> Unless you can tell me how to somehow modify the installation of Weechat to also include this one Python package.

    pkgs.weechat.override { extraBuildInputs = [ pkgs.python3Packages.pync ]; };
  };

That Nix expression defines a package meaning 'just like Weechat, but with pync in the build paths'. There are examples with links to more docs on the unofficial NixOS wiki: https://nixos.wiki/wiki/Weechat

Looking at the Nixpkgs docs, you might prefer a different override expression. Several examples are provided: https://nixos.org/manual/nixpkgs/stable/#sec-weechat

I understand that this may still feel like 'making a simple thing hard', but it does, I hope, answer your question. Modifying existing packages in just a few lines of code in order to include additional dependencies is very much part of Nix's paradigm. :)

There's a way you could write all that inline to install it with nix-env, too, but I can't be arsed.