Configuring programs and services from unstable channel in NixOS

I know that I can install a package from the unstable channel like this: { config, pkgs, ... }: let unstable-pkgs = import {}; in { environment.systemPackages = [ unstable-pkgs.chromium ] } This shows that import {}; stands in for pkgs in my configuration, but what if I want to do the same thing for programs or services?