Since this is an interesting and different request, I decided that it deserves an answer. So, to start, you can find an unprotected map,"Tower Defence" with an example building at http://www.battle.net/war3/maps/war3xbonusmaps2.shtml
From the map, you can gather that the building is based on a hero, but with some modified fields: Movement - Speed Base = 0 Stats - Is a Building = True
Also, the building has a fake copy. This is for the trigger "Player 0x High Exp", found in the "Increase Exp" folder to replace the old building with the new hero. Cheers