How to override packages in a nix derivation?

Question

While developing a nix package, my default.nix looks like this:

let
  pkgs = import <nixpkgs> {
    config = { ... }
  }
in pkgs.path.to.derivation

But when turning that into an actual package, the pkgs comes in as an argument. But adding a config to that doesn't seem to work:

{ pkgs, ... }:
let
  pkgs_ = pkgs // { config = { ... } };
in pkgs_.path.to.derivation

How do I add the same config to an already existing package set?