configuration.nix

let 
  myOverlay = (self: super: {
    # Some totally new package
    my-package-name = super.callPackage /path/to/my/pkg/default.nix;
    # And you can override existing keys if you want, or refer to packages in the parent overlay
    vim = super.neovim;
  });
in
{
  nixpkgs.overlays = [ myOverlay ];
  environment.systemPackages = with pkgs; [ my-package-name vim ];
  # ...
}