nixpkgs/nixos/doc/manual/development/importing-modules.section.md

Importing Modules

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

{ config, lib, pkgs, ... }:

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

  services.exampleModule.enable = true;
}

The environment variable NIXOS_EXTRA_MODULE_PATH 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:

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

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

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

{
  # No `imports` needed

  services.exampleModule1.enable = true;
}