You can set a waypoint destination to travel to, be it a turbo lift, jefry tube (sp?), or whatever. Then load another interior map to represent the next level.
You can also place a door object over one of the actual doors in a corridor, and make that an interactable object. If you put your next objective on the 'lower level' map, you'll spawn a map transition point. That way, you get the glowing object and the sense that you're activating a turbolift or the like to move between maps.