||I think alot depends on how much you are going to move the bus. I have seen many that were being used as a semi- stationary home and the owners added a second story using standard wooden construction components.(2x4's etc.)If you were planning on moving the bus frequently, I would think a metal frame at least would have to be fabricated and welded to the bus and then it could be covered with sheet metal or you could use plywood/osb, then cover the wood with fiberglass panels like a factory-built RV. The VW micro bus idea has been done frequently too. Alternately you could use any van for the upper floor. Watch the height of the bus and make sure you stay under 13.6 feet, or you may lose your addition at the first low overpass.
:0( Hope this helps, Mike