configuration.nix

{ ...}:

{
  #...

  nixpkgs.overlays = [
    (import ./overlay.nix)
  ];

  #...
}

overlay.nix

self: super: {
  # Desktop item that runs `ssh-add`
  ssh-add-desktop = super.makeDesktopItem {
    name = "ssh-add";
    exec = "${super.openssh}/bin/ssh-add";
    comment = "Add identities to the SSH authentication agent";
    desktopName = "Add SSH Identities";
    genericName = "SSH";
    categories = "Application;System;";
  };

  # Packaging nixos-shell
  nixos-shell = super.stdenv.mkDerivation {
    name = "nixos-shell";
    src = super.fetchFromGitHub {
      owner = "wavewave";
      repo = "nixos-shell";
      rev = "1e896190f7971e963efed6c3db45c6783dc9032b";
      fetchSubmodules = true;
      sha256 = "1sjp9vawybiqr4pa6aryqgz17vixnywml17x4maiirw11v4marhb";
    };
    buildInputs = with super; [ go ];
    phases = [ "buildPhase" ];
    buildPhase = ''
      mkdir -p $out/bin
      GOPATH=$src go build -o $out/bin/nixos-shell nixos-shell
    '';
  };
}