ok, un example:

{
  description = "Cinny + gst";
  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
  outputs =
    { self, nixpkgs }:
    let
      system = "x86_64-linux";
      pkgs = nixpkgs.legacyPackages.${system};
    in
    {
      packages.${system}.default = pkgs.cinny.overwriteAttrs (super: {
        propagatedBuildInputs = (super.propagatedBuildInputs or [ ]) ++ [
          pkgs.gst_all_1.gstreamer
          pkgs.gst_all_1.gst-plugins-base
          pkgs.gst_all_1.gst-plugins-good
          pkgs.gst_all_1.gst-plugins-bad
        ];
      });
    };
}