nixpkgs/nixos/doc/manual/development/importing-modules.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-importing-modules">
 <title>Importing Modules</title>

 <para>
  Sometimes NixOS modules need to be used in configuration but exist outside of
  Nixpkgs. These modules can be imported:
 </para>

<programlisting>
{ config, lib, pkgs, ... }:

{
  imports =
    [ # Use a locally-available module definition in
      # ./example-module/default.nix
        ./example-module
    ];

  services.exampleModule.enable = true;
}
</programlisting>

 <para>
  The environment variable <literal>NIXOS_EXTRA_MODULE_PATH</literal> is an
  absolute path to a NixOS module that is included alongside the Nixpkgs NixOS
  modules. Like any NixOS module, this module can import additional modules:
 </para>

<programlisting>
# ./module-list/default.nix
[
  ./example-module1
  ./example-module2
]
</programlisting>

<programlisting>
# ./extra-module/default.nix
{ imports = import ./module-list.nix; }
</programlisting>

<programlisting>
# NIXOS_EXTRA_MODULE_PATH=/absolute/path/to/extra-module
{ config, lib, pkgs, ... }:

{
  # No `imports` needed

  services.exampleModule1.enable = true;
}
</programlisting>
</section>