NixOS configuration overlays

configuration.nix

{ config, pkgs, ... }:

let
  # Import unstable channel.
  # sudo nix-channel --add http://nixos.org/channels/nixpkgs-unstable nixpkgs-unstable
  # sudo nix-channel --update nixpkgs-unstable
  unstable = import <nixpkgs-unstable> {};
in

{
  environment.systemPackages =
    [ pkgs.hello  # regular channel
      unstable.spotify  # newer version from nixpkgs-unstable
    ];

  virtualisation.libvirtd.enable = true;

  nixpkgs.overlays =
    [ (self: super:
      {
        # override with newer version from nixpkgs-unstable
        qemu = unstable.qemu;

        # custom package that depends on hello from nixpkgs-unstable
        foo = super.callPackage ./pkgs/foo { inherit (unstable) hello; };
      })
    ];
}

It might be possible to bring in unstable.qemu with dependencies satisfied from pkgs using override attribute of package.