nixpath

configuration.nix

{

  nix.nixPath = [
    "nixos-config=/etc/nixos/configuration.nix"
    "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs"
    "nixpkgs-unstable=/nix/var/nix/profiles/per-user/root/channels/nixos-unstable/nixpkgs"
    "nixpkgs-fork=/home/betaboon/src/nix/nixpkgs-fork"
    "/nix/var/nix/profiles/per-user/root/channels"
  ];

}

some-other-configuration.nix

let
  pkgs-unstable = import <nixpkgs-unstable> { };
  pkgs-fork = import <nixpkgs-fork> { };
in {

  environment.systemPackages = [
    pkgs-unstable.foobar
  ];

};