Why can't you have your Navigation/Polygon as a part of the world your enemies are pathfinding around? I'm not even sure how it would work to have the navigation on the enemies..
All you need is to have some function on your enemy that references their world (assume the enemy is a direct child of the world), and returns the path from their position to wherever they're aiming for.
(on the world)
func find_path_to_player(from_pos, to_pos):
(on the enemy)
path = get_parent().find_path(self.get_pos(), to_point)
From there, the enemy gets its path and figures out what to do with it from there.