foo.nix

{ config, pkgs, lib, ... }:
# all $HOME and user configurations
with lib;
let
  extraPkgs = import <nixpkgs> { config={}; overlays=[]; };
  homeManager = extraPkgs.fetchFromGitHub {
    owner = "rycee";
    repo = "home-manager";
    rev = "gitrev";
    sha256 = "hash";
  };
in
  {
    imports = [
      # make home-manager available https://nixos.wiki/wiki/Home_Manager
      "${homeManager}/nixos"
    ];

  }