nixpkgs/nixos/doc/manual/configuration/adding-custom-packages.xml

<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-custom-packages">

<title>Adding Custom Packages</title>

<para>It’s possible that a package you need is not available in NixOS.
In that case, you can do two things.  First, you can clone the Nixpkgs
repository, add the package to your clone, and (optionally) submit a
patch or pull request to have it accepted into the main Nixpkgs
repository.  This is described in detail in the <link
xlink:href="http://nixos.org/nixpkgs/manual">Nixpkgs manual</link>.
In short, you clone Nixpkgs:

<screen>
$ git clone git://github.com/NixOS/nixpkgs.git
$ cd nixpkgs
</screen>

Then you write and test the package as described in the Nixpkgs
manual.  Finally, you add it to
<literal>environment.systemPackages</literal>, e.g.

<programlisting>
environment.systemPackages = [ pkgs.my-package ];
</programlisting>

and you run <command>nixos-rebuild</command>, specifying your own
Nixpkgs tree:

<screen>
# nixos-rebuild switch -I nixpkgs=/path/to/my/nixpkgs</screen>

</para>

<para>The second possibility is to add the package outside of the
Nixpkgs tree.  For instance, here is how you specify a build of the
<link xlink:href="http://www.gnu.org/software/hello/">GNU Hello</link>
package directly in <filename>configuration.nix</filename>:

<programlisting>
environment.systemPackages =
  let
    my-hello = with pkgs; stdenv.mkDerivation rec {
      name = "hello-2.8";
      src = fetchurl {
        url = "mirror://gnu/hello/${name}.tar.gz";
        sha256 = "0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6";
      };
    };
  in
  [ my-hello ];
</programlisting>

Of course, you can also move the definition of
<literal>my-hello</literal> into a separate Nix expression, e.g.
<programlisting>
environment.systemPackages = [ (import ./my-hello.nix) ];
</programlisting>
where <filename>my-hello.nix</filename> contains:
<programlisting>
with import &lt;nixpkgs> {}; # bring all of Nixpkgs into scope

stdenv.mkDerivation rec {
  name = "hello-2.8";
  src = fetchurl {
    url = "mirror://gnu/hello/${name}.tar.gz";
    sha256 = "0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6";
  };
}
</programlisting>

This allows testing the package easily:
<screen>
$ nix-build my-hello.nix
$ ./result/bin/hello
Hello, world!
</screen>

</para>

</section>