Add dependency to a derivation

I am using haskell.nix in a project. In general, it works very well. However, one my tests have a dependency on ephemeralpg. That type of dependency is usually straigh forward, you just add the dependency to the buildInputs and you are done. In this case, none of that is exposed from the haskell-nix infrastructure. I don’t want to loose that infrastructure, but I do want to be able to add non-haskell dependencies when they arise. Below is the derivation in question.

{ nixpkgs ? <nixpkgs>, haskellCompiler ? "ghc865" }:
  spec = builtins.fromJSON (builtins.readFile ./haskell-nix-src.json);
  haskell-nix-src = (import nixpkgs { }).fetchgit {
    name = "haskell-lib";
    inherit (spec) url rev sha256 fetchSubmodules;
in with import nixpkgs (import haskell-nix-src);
pkgs.haskell-nix.cabalProject {
  src = pkgs.haskell-nix.haskellLib.cleanGit { src = ../.; };
  ghc = pkgs.buildPackages.pkgs.haskell-nix.compiler.${haskellCompiler};
  pkg-def-extras = [];
   modules = [
  #   # Specific package overrides would go here for example:
  #   packages.cbors.package.ghcOptions = "-Werror";
  #   packages.cbors.patches = [ ./one.patch ];
  #   packages.cbors.flags.optimize-gmp = false;
  #   # It may be better to set flags in `cabal.project` instead
  #   # (`plan-to-nix` will include them as defaults).

You end up with our build outputs in `project.components.<component-name>`. I have made a few attempts at modifying (copying, adding, returning the new one) the derivation before it gets returned, but haven't been successful in those attempts.

Hi sorry for necroposting, but have you ever found an answer to this question?