Export multiple levels in one map

I think it would be good for smaller 20x20 maps to be able export all of the different floors on one large map. Say you have a Basement, groundfloor, second floor, and roof. That would be 4 20x20 maps, plus border. If the border is just a square or two between the maps, then your overall map would be 52x52. Which is reasonable. So rather than having to move your players from map to map to map when they go up and down floors, you can keep them all on the same space, and just swing them to different sections of the map.