nix-overlay

configuration.nix

{ config, pkgs, ... }:

{

  nixpkgs.overlays = [
    (import ./my-overlay.nix)
  ];
  
  environment.systemPackages = [
    myPackage
  ];

}

my-overlay.nix

self: super:

{

  myPackage = super.callPackage ./my-package.nix;
  
}

my-package.nix

{ stdenv, ... }:

stdenv.mkDerivation rec {

  name = "my-package-${version}";
  version = "1.0";

  src = /path/to/my/packages/source;
  
}