if you wanted to do a tunnel for instance, you could place a piece of level geometry/prefab etc... anything that looked like the entrance to a tunnel on the heightmap. then place an invisible entity with a level change property attached to it in front of the entrance. once the character runs into this invisible trigger, he can be transported to a tunnel level you create in wed (like any other). inside the tunnel level, you can have a similar trigger set up to return to heightmap area..... just a thought