nixpkgs/modules/misc/passthru.nix

# This module allows you to export something from configuration
# Use case: export kernel source expression for ease of configuring 

{config, pkgs, ...}:

let 

options = {
  passthru = pkgs.lib.mkOption {
    description = ''
      This attribute set will be exported as a system attribute.
      You can put whatever you want here.
    '';
  };
};

in 

{
  require = options;
}