diff --git a/.travis.yml b/.travis.yml
index c2ec15e..3667bf2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,6 +14,7 @@ env:
   - EXAMPLE=rss2email
   - EXAMPLE=scipy
   - EXAMPLE=tornado
+  - EXAMPLE=connexion
 #  - EXAMPLE=vulnix
 matrix:
   exclude:
diff --git a/README.rst b/README.rst
index c37455a..5a11a6c 100644
--- a/README.rst
+++ b/README.rst
@@ -180,6 +180,13 @@ dependency of ``execnet``::
 
 This was you can add or remove any python package.
 
+Including overrides
+^^^^^^^^^^^^^^^^^^^
+
+Additional to a autogenerated ``requirements_overrides.nix`` file you
+can include additional overrides files via the ``-O`` command line
+argument.  These overrides will be included the same way as your
+``requirements_overrides.nix``.
 
 Creating default.nix for you project
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/default.nix b/default.nix
index 3c92ed6..34179a5 100644
--- a/default.nix
+++ b/default.nix
@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, zip, makeWrapper, nix, nix-prefetch-scripts
+{ stdenv, fetchurl, zip, makeWrapper, nix, nix-prefetch-git, nix-prefetch-hg
 , pythonPackages
 , src ? { outPath = ./.; name = "pypi2nix"; }
 }:
@@ -26,7 +26,7 @@ in stdenv.mkDerivation rec {
   ];
   buildInputs = [
     pythonPackages.python pythonPackages.flake8
-    zip makeWrapper nix.out nix-prefetch-scripts
+    zip makeWrapper nix.out nix-prefetch-git nix-prefetch-hg
   ];
   doCheck = true;
   sourceRoot = ".";
@@ -48,8 +48,8 @@ in stdenv.mkDerivation rec {
 
   patchPhase = ''
     sed -i -e "s|default='nix-shell',|default='${nix.out}/bin/nix-shell',|" $out/pkgs/pypi2nix/cli.py
-    sed -i -e "s|nix-prefetch-git|${nix-prefetch-scripts}/bin/nix-prefetch-git|" $out/pkgs/pypi2nix/stage2.py
-    sed -i -e "s|nix-prefetch-hg|${nix-prefetch-scripts}/bin/nix-prefetch-hg|" $out/pkgs/pypi2nix/stage2.py
+    sed -i -e "s|nix-prefetch-git|${nix-prefetch-git}/bin/nix-prefetch-git|" $out/pkgs/pypi2nix/stage2.py
+    sed -i -e "s|nix-prefetch-hg|${nix-prefetch-hg}/bin/nix-prefetch-hg|" $out/pkgs/pypi2nix/stage2.py
   '';
 
   commonPhase = ''
@@ -69,7 +69,9 @@ in stdenv.mkDerivation rec {
   '';
 
   installPhase = commonPhase + ''
-    wrapProgram $out/bin/pypi2nix --prefix PYTHONPATH : "$PYTHONPATH"
+    wrapProgram $out/bin/pypi2nix \
+        --prefix PYTHONPATH : "$PYTHONPATH" \
+        --prefix PATH : "${nix-prefetch-git}/bin:${nix-prefetch-hg}/bin"
   '';
 
   shellHook = ''
diff --git a/examples/Makefile b/examples/Makefile
index bc32b39..d46eded 100644
--- a/examples/Makefile
+++ b/examples/Makefile
@@ -1,6 +1,8 @@
 OS := $(shell uname)
 PYPI2NIX=./pypi2nix/bin/pypi2nix
 NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixpkgs-unstable.tar.gz
+NIX_BUILD=nix-build
+NIX_SHELL=nix-shell
 
 
 ifeq ($(OS), Darwin)
@@ -19,7 +21,8 @@ endif
 	scipy \
 	pillow \
 	tornado \
-	vulnix
+	vulnix \
+	connexion
 
 all: \
 	awscli_and_requests \
@@ -31,7 +34,8 @@ all: \
 	scipy \
 	pillow \
 	tornado \
-	vulnix
+	vulnix \
+	connexion
 
 clear: \
 	awscli_and_requests-clear \
@@ -44,7 +48,8 @@ clear: \
 	scipy-clear \
 	pillow-clear \
 	tornado-clear \
-	vulnix-clear
+	vulnix-clear \
+	connexion-clear
 
 
 pypi2nix-clear:
@@ -52,13 +57,13 @@ pypi2nix-clear:
 
 $(PYPI2NIX): pypi2nix-clear
 	echo "building pypi2nix ..."
-	nix-build ../release.nix -A build.x86_64-linux -o pypi2nix
+	$(NIX_BUILD) ../release.nix -A build.x86_64-linux -o pypi2nix
 
 pillow: pillow.nix
 	echo "building and testing pillow..."
-	nix-build pillow.nix -o pillow -A interpreter --show-trace
+	$(NIX_BUILD) pillow.nix -o pillow -A interpreter --show-trace
 	./pillow/bin/python -c 'import PIL'
-	nix-shell pillow.nix -A interpreter --run "python -c 'import PIL'" 
+	$(NIX_SHELL) pillow.nix -A interpreter --run "python -c 'import PIL'" 
 
 pillow.nix: pillow.txt $(PYPI2NIX)
 	echo "generating pillownix expressions ..."
@@ -82,9 +87,9 @@ pillow-clear:
 
 empy: empy.nix
 	echo "building and testing empy ..."
-	nix-build empy.nix -o empy -A interpreter --show-trace
+	$(NIX_BUILD) empy.nix -o empy -A interpreter --show-trace
 	./empy/bin/python -c 'import em'
-	nix-shell empy.nix -A interpreter --run "python -c 'import em'"
+	$(NIX_SHELL) empy.nix -A interpreter --run "python -c 'import em'"
 
 empy.nix: empy.txt $(PYPI2NIX)
 	echo "generating empy nix expressions ..."
@@ -107,11 +112,11 @@ empy-clear:
 
 lektor: lektor.nix
 	echo "building and testing lektor ..."
-	nix-build lektor.nix -o lektor -A interpreter --show-trace
+	$(NIX_BUILD) lektor.nix -o lektor -A interpreter --show-trace
 	./lektor/bin/python -c 'import lektor'
 	./lektor/bin/lektor --help
-	nix-shell lektor.nix -A interpreter --run "python -c 'import lektor'"
-	nix-shell lektor.nix -A interpreter --run "lektor --help"
+	$(NIX_SHELL) lektor.nix -A interpreter --run "python -c 'import lektor'"
+	$(NIX_SHELL) lektor.nix -A interpreter --run "lektor --help"
 
 lektor.nix: lektor.txt $(PYPI2NIX)
 	echo "generating lektor nix expressions ..."
@@ -136,11 +141,11 @@ lektor-clear:
 
 rss2email: rss2email.nix
 	echo "building and testing rss2email ..."
-	nix-build rss2email.nix -o rss2email -A interpreter --show-trace
+	$(NIX_BUILD) rss2email.nix -o rss2email -A interpreter --show-trace
 	./rss2email/bin/python -c 'import rss2email'
 	./rss2email/bin/r2e --help
-	nix-build rss2email.nix -A interpreter --run "python -c 'import rss2email'"
-	nix-build rss2email.nix -A interpreter --run "r2e --help"
+	$(NIX_BUILD) rss2email.nix -A interpreter --run "python -c 'import rss2email'"
+	$(NIX_BUILD) rss2email.nix -A interpreter --run "r2e --help"
 
 rss2email.nix: rss2email.txt $(PYPI2NIX)
 	echo "generating rss2email nix expressions ..."
@@ -163,12 +168,12 @@ rss2email-clear:
 
 awscli_and_requests: awscli_and_requests.nix
 	echo "building and testing awscli and requests library ..."
-	nix-build awscli.nix -o awscli_and_requests -A interpreter --show-trace
+	$(NIX_BUILD) awscli.nix -o awscli_and_requests -A interpreter --show-trace
 	./awscli_and_requests/bin/python -c 'import awscli; import requests'
 	PAGER=none ./awscli_and_requests/bin/aws help
-	nix-shell awscli.nix -A interpreter --run "python -c 'import awscli'"
-	nix-shell awscli.nix -A interpreter --run "python -c 'import awscli; import requests'"
-	nix-shell awscli.nix -A interpreter --run "PAGER=none aws help"
+	$(NIX_SHELL) awscli.nix -A interpreter --run "python -c 'import awscli'"
+	$(NIX_SHELL) awscli.nix -A interpreter --run "python -c 'import awscli; import requests'"
+	$(NIX_SHELL) awscli.nix -A interpreter --run "PAGER=none aws help"
 
 awscli_and_requests.nix: awscli_and_requests.txt $(PYPI2NIX)
 	echo "generating awscli and requests nix expressions ..."
@@ -190,8 +195,8 @@ awscli_and_requests-clear:
 vulnix: vulnix-clear $(PYPI2NIX)
 	echo "building and testing vulnix..."
 	git clone https://github.com/garbas/vulnix vulnix-src
-	cd vulnix-src && nix-shell update.nix --argstr pypi2nix "./../$(PYPI2NIX)"
-	NIX_PATH=$(NIX_PATH) nix-build vulnix-src/default.nix --arg pkgs "import <nixpkgs> {}" -o vulnix
+	cd vulnix-src && $(NIX_SHELL) update.nix --argstr pypi2nix "./../$(PYPI2NIX)"
+	NIX_PATH=$(NIX_PATH) $(NIX_BUILD) vulnix-src/default.nix --arg pkgs "import <nixpkgs> {}" -o vulnix
 	./vulnix/bin/vulnix --help
 
 vulnix-clear:
@@ -201,9 +206,9 @@ vulnix-clear:
 
 scipy: scipy.nix
 	echo "building and testing scipy..."
-	nix-build scipy.nix -o scipy -A interpreter --show-trace
+	$(NIX_BUILD) scipy.nix -o scipy -A interpreter --show-trace
 	./scipy/bin/python -c 'import scipy'
-	nix-shell scipy.nix -A interpreter --run "python -c 'import scipy'"
+	$(NIX_SHELL) scipy.nix -A interpreter --run "python -c 'import scipy'"
 
 scipy.nix: scipy.txt $(PYPI2NIX)
 	echo "generating scipy.nix expressions ..."
@@ -221,14 +226,14 @@ scipy-clear:
 
 tornado: tornado.nix
 	echo "building and testing tornado..."
-	nix-build tornado.nix -o tornado -A interpreter --show-trace
+	$(NIX_BUILD) tornado.nix -o tornado -A interpreter --show-trace --fallback
 	./tornado/bin/python -c 'import tornado'
-	nix-shell tornado.nix -A interpreter --run "python -c 'import tornado'"
+	$(NIX_SHELL) tornado.nix -A interpreter --run "python -c 'import tornado'"
 	if ./string_in_file "69253c820df473407c562a227d0ba36df25018ab" tornado.nix ; then true ; else echo "ERROR: Revision '69253c820df473407c562a227d0ba36df25018ab' not found in tornado.nix!" ; exit 123 ; fi
 
 tornado.nix: tornado.txt $(PYPI2NIX)
 	echo "generating tornado.nix expressions ..."
-	$(PYPI2NIX) -v --basename tornado -r tornado.txt -I $(NIX_PATH) -V "3.5"
+	$(PYPI2NIX) -v --basename tornado -r tornado.txt -I $(NIX_PATH) -V "3.5" -O 'https://raw.githubusercontent.com/garbas/nixpkgs-python/master/overrides.nix' -O 'git+https://github.com/garbas/nixpkgs-python.git#path=overrides.nix&rev=28ecf4bc8cf5e718862159eb1f324ddcf227dfef'
 
 tornado.txt: tornado-clear
 	echo "-e git+git://github.com/tornadoweb/tornado.git@69253c820df473407c562a227d0ba36df25018ab#egg=tornado" > tornado.txt
@@ -242,9 +247,9 @@ tornado-clear:
 
 ldap: ldap.nix
 	echo "building and testing ldap..."
-	nix-build ldap.nix -o ldap -A interpreter --show-trace
+	$(NIX_BUILD) ldap.nix -o ldap -A interpreter --show-trace
 	./ldap/bin/python -c 'import ldap'
-	nix-shell ldap.nix -A interpreter --run "python -c 'import ldap'"
+	$(NIX_SHELL) ldap.nix -A interpreter --run "python -c 'import ldap'"
 
 ldap.nix: ldap.txt $(PYPI2NIX)
 	echo "generating ldap.nix expressions ..."
@@ -269,9 +274,9 @@ ldap-clear:
 
 mercurial: mercurial.nix
 	echo "building and testing mercurial..."
-	nix-build mercurial.nix -o mercurial -A interpreter --show-trace -j 1
+	$(NIX_BUILD) mercurial.nix -o mercurial -A interpreter --show-trace -j 1
 	./mercurial/bin/python -c 'import flake8'
-	nix-shell mercurial.nix -A interpreter --run "python -c 'import flake8'"
+	$(NIX_SHELL) mercurial.nix -A interpreter --run "python -c 'import flake8'"
 	if ./string_in_file "a209fb6" mercurial.nix ; then true ; else echo "ERROR: Revision 'a209fb6' not found in tornado_generated.nix!"; exit 123; fi
 
 mercurial.nix: mercurial.txt $(PYPI2NIX)
@@ -289,3 +294,23 @@ mercurial-clear:
 	rm -f mercurial.txt
 	rm -f mercurial.nix
 	rm -f mercurial_frozen.txt
+
+connexion: connexion.nix
+	echo "building and testing connexion..."
+	$(NIX_BUILD) connexion.nix -o connexion -A interpreter --show-trace -j 1
+	./connexion/bin/python -c 'import connexion'
+	$(NIX_SHELL) connexion.nix -A interpreter --run "python -c 'import connexion'"
+
+connexion.nix: connexion.txt $(PYPI2NIX)
+	echo "generating connexion.nix expressions ..."
+	$(PYPI2NIX) -v --basename connexion -r connexion.txt -I $(NIX_PATH) -V "2.7" -e vcversioner -O "git+https://github.com/garbas/nixpkgs-python.git#path=overrides.nix"
+
+
+connexion.txt: connexion-clear
+	echo "connexion" > connexion.txt
+
+connexion-clear:
+	rm -f connexion
+	rm -f connexion.txt
+	rm -f connexion.nix
+	rm -f connexion_frozen.txt
diff --git a/examples/connexion_override.nix b/examples/connexion_override.nix
new file mode 100644
index 0000000..9aa93a4
--- /dev/null
+++ b/examples/connexion_override.nix
@@ -0,0 +1,9 @@
+{ pkgs, python }:
+
+self: super: {
+
+  "jsonschema" = python.overrideDerivation super."jsonschema" (old: {
+    propagatedBuildInputs = old.propagatedBuildInputs ++ [self.vcversioner];
+  });
+
+}
diff --git a/src/pypi2nix/cli.py b/src/pypi2nix/cli.py
index 6a4dfa7..007a22d 100644
--- a/src/pypi2nix/cli.py
+++ b/src/pypi2nix/cli.py
@@ -4,6 +4,7 @@
 import shutil
 import tempfile
 
+import pypi2nix.overrides
 import pypi2nix.stage0
 import pypi2nix.stage1
 import pypi2nix.stage2
@@ -96,6 +97,13 @@
               help=u'Extra Python dependencies needed before the installation'
                    u'to build wheels.'
               )
+@click.option('-O', '--overrides',
+              multiple=True,
+              required=False,
+              type=pypi2nix.overrides.OVERRIDES_URL,
+              help=u'Extra expressions that override generated expressions ' +
+                   u'for specific packages',
+              )
 def main(version,
          verbose,
          nix_shell,
@@ -110,6 +118,7 @@ def main(version,
          buildout,
          editable,
          setup_requires,
+         overrides
          ):
     """SPECIFICATION should be requirements.txt (output of pip freeze).
     """
@@ -319,6 +328,7 @@ def handle_requirements_file(project_dir, requirements_file):
         enable_tests=enable_tests,
         python_version=pypi2nix.utils.PYTHON_VERSIONS[python_version],
         current_dir=current_dir,
+        common_overrides=overrides,
     )
 
     click.echo('')
diff --git a/src/pypi2nix/overrides.py b/src/pypi2nix/overrides.py
new file mode 100644
index 0000000..37c013a
--- /dev/null
+++ b/src/pypi2nix/overrides.py
@@ -0,0 +1,127 @@
+import json
+import subprocess
+from urllib.parse import urldefrag, urlparse
+
+import click
+
+from .utils import cmd
+
+
+class UnsupportedUrlError(Exception):
+    pass
+
+
+class OverridesFile(object):
+    def __init__(self, path):
+        self.path = path
+
+    expression_template = 'import %(path)s { inherit pkgs python ; }'
+
+    def nix_expression(self):
+        return self.expression_template % dict(path=self.path)
+
+
+class OverridesUrl(object):
+    def __init__(self, url):
+        self.url = url
+
+    expression_template = (
+        'let src = pkgs.fetchurl { ' +
+        'url = %(url)s; sha256 = "%(sha_string)s"; ' +
+        '}; in ' +
+        'import "${src}" { inherit pkgs python ; }'
+    )
+
+    def nix_expression(self):
+        command = 'nix-prefetch-url {url}'.format(
+            url=self.url
+        )
+
+        return_code, output = cmd(
+            command, verbose=False, stderr=subprocess.DEVNULL
+        )
+        sha_sum = output.strip()
+        if len(sha_sum) != 52 or return_code != 0:
+            raise click.ClickException(
+                'Could not determin hash for url %{url}s' % dict(
+                    url=self.url
+                )
+            )
+        return self.expression_template % dict(
+            url=self.url,
+            sha_string=sha_sum
+        )
+
+
+class OverridesGit(object):
+    def __init__(self, repo_url, path, rev=None):
+        self.repo_url = repo_url
+        self.path = path
+        self.rev = rev
+
+    expression_template = (
+        'let src = pkgs.fetchgit { ' +
+        'url = "%(url)s"; ' +
+        'sha256 = "%(sha256)s"; ' +
+        'rev = "%(rev)s"; ' +
+        '} ; in import "${src}/%(path)s" { inherit pkgs python; }'
+    )
+
+    def nix_expression(self):
+        command = 'nix-prefetch-git %(url)s' % dict(
+            url=self.repo_url
+        )
+
+        if self.rev is not None:
+            command += ' --rev %(rev)s' % dict(rev=self.rev)
+
+        return_code, output = cmd(
+            command, verbose=False, stderr=subprocess.DEVNULL
+        )
+
+        if return_code != 0:
+            raise click.ClickException(
+                'Could not fetch git repository at %(url)s' % dict(
+                    url=self.repo_url
+                )
+            )
+        repo_data = json.loads(output)
+        return self.expression_template % dict(
+            url=repo_data['url'],
+            sha256=repo_data['sha256'],
+            rev=repo_data['rev'],
+            path=self.path
+        )
+
+
+def url_to_overrides(url_string):
+    url = urlparse(url_string)
+    if url.scheme == 'file' or '':
+        return OverridesFile(url.path)
+    elif url.scheme == 'http' or url.scheme == 'https':
+        return OverridesUrl(url.geturl())
+    elif url.scheme.startswith('git+'):
+        fragments = dict(
+            map(lambda x: x.split('='), url.fragment.split('&'))
+        )
+        return OverridesGit(
+            repo_url=urldefrag(url.geturl()[4:])[0],
+            path=fragments['path'],
+            rev=fragments.get('rev', None),
+        )
+    else:
+        raise UnsupportedUrlError('Cannot handle common overrides url %s' %
+                                  url_string)
+
+
+class OverridesUrlParam(click.ParamType):
+    name = 'url'
+
+    def convert(self, value, param, ctx):
+        try:
+            return url_to_overrides(value)
+        except UnsupportedUrlError as e:
+            self.fail(str(e), param, ctx)
+
+
+OVERRIDES_URL = OverridesUrlParam()
diff --git a/src/pypi2nix/stage3.py b/src/pypi2nix/stage3.py
index 42ea67c..9079b45 100644
--- a/src/pypi2nix/stage3.py
+++ b/src/pypi2nix/stage3.py
@@ -1,5 +1,6 @@
-import sys
 import os
+import sys
+
 import click
 
 
@@ -74,8 +75,17 @@
 %(generated_package_nix)s
   };
   overrides = import %(overrides_file)s { inherit pkgs python; };
-
-in python.withPackages (fix' (extends overrides generated))
+  commonOverrides = [
+%(common_overrides)s
+  ];
+
+in python.withPackages
+   (fix' (pkgs.lib.fold
+            extends
+            generated
+            ([overrides] ++ commonOverrides)
+         )
+   )
 '''
 
 GENERATED_NIX = '''# generated using pypi2nix tool (version: %s)
@@ -123,6 +133,7 @@ def main(packages_metadata,
          enable_tests,
          python_version,
          current_dir,
+         common_overrides=[],
          ):
     '''Create Nix expressions.
     '''
@@ -199,6 +210,11 @@ def main(packages_metadata,
 
     overrides = OVERRIDES_NIX % ""
 
+    common_overrides_expressions = [
+        '    (' + override.nix_expression() + ')'
+        for override in common_overrides
+    ]
+
     default = DEFAULT_NIX % dict(
         version=version,
         command_arguments=' '.join(sys.argv[1:]),
@@ -211,6 +227,7 @@ def main(packages_metadata,
         overrides_file='.' + overrides_file[len(current_dir):],
         enable_tests=str(enable_tests).lower(),
         generated_package_nix=generated,
+        common_overrides='\n'.join(common_overrides_expressions),
     )
 
     if not os.path.exists(overrides_file):
diff --git a/src/pypi2nix/utils.py b/src/pypi2nix/utils.py
index 133ff93..9403522 100644
--- a/src/pypi2nix/utils.py
+++ b/src/pypi2nix/utils.py
@@ -38,7 +38,7 @@ def safe(string):
     return string.replace('"', '\\"')
 
 
-def cmd(command, verbose=False):
+def cmd(command, verbose=False, stderr=subprocess.STDOUT):
 
     if isinstance(command, str):
         command = shlex.split(command)
@@ -49,7 +49,7 @@ def cmd(command, verbose=False):
     p = subprocess.Popen(
         command,
         stdout=subprocess.PIPE,
-        stderr=subprocess.STDOUT,
+        stderr=stderr,
     )
 
     out = []