You can export the variable and then set it in the editor using the following syntax in GDScript:
export (type_here) var variable_name_here : optional_type_here = default_value_here
# working example:
export (NodePath) var path_to_node : NodePath = null
I'm not sure if that is what you are looking for though, but maybe? For a PackedScene, something like this might be what you are looking for (untested):
# for a filename string:
export (String, FILE) var file_name = ""
# for a PackedScene
export (PackedScene) var scene_file = null