This is adapted from my post:  https://matthewbauer.us/blog/channel-changing.html.

I am still undecided whether this is a good idea but I definitely
think it’s cool. This adds some top-level attributes for old releases
of NixOS/Nixpkgs. With them you can combine old versions of stuff in
one package, fairly easily:

{ pkgs }:
pkgs.impala.stdenv.mkDerivation {
  name = "hello";
  src = pkgs.impala.fetchurl { ... };
  buildInput = [ pkgs.gorilla.firefox
                 pkgs.emu.libedit ];
}