default.nix

{ pkgs ? import <nixpkgs> {} }:

let 
  mypkg = pkgs.callPackage ./pkg.nix {};
in mypkg.overrideAttrs(oldAttrs: rec {
  config = "NEW config";
})

pkg.nix

{ pkgs ? import <nixpkgs> {} }:

let
  config = "thisismyconfig";
in pkgs.stdenv.mkDerivation {
  name = "test";
  phases = "buildPhase";

  config = config;

  buildPhase = ''
  echo ${config} > $out
  '';

}

test2-default.nix

{ pkgs ? import <nixpkgs> {} }:

let 
  mypkg = pkgs.callPackage ./test2-pkg.nix {};
in pkgs.callPackage mypkg.build { config = "lal"; }

test2-pkg.nix

{ pkgs ? import <nixpkgs> {} }:

let
  config = "thisismyconfig";
  build = cfg: pkgs.stdenv.mkDerivation {
    name = "test";
    phases = "buildPhase";

    buildPhase = ''
    echo ${cfg.config} > $out
    '';
  };

in {
  "config" = config;
  "build" = pkgs.callPackage build { "config" = config; };
}