it’s a language specific package, so you need to add it to the related directories, for python this is:
top-level/python-modules.nix # for entering attribute e.g. pythonPackages.package
pkgs/development/python-modules/<package>/default.nix # nix expression
pretty standard module addition example: pythonPackages.mpld3: init at 0.3 by unode · Pull Request #70457 · NixOS/nixpkgs · GitHub