Interesting idea. The way it works right now is defense mode spawns one of the predefined buildings (megastore, bar, hospital, or mansion), it’s certainly possible to make new defense-mode specific buildings.
There are discussions of adding new buildings to the game in several threads, such as http://smf.cataclysmdda.com/index.php?topic=941.0, and vache is working on a standalone building editor that will let you layout new buildings. It outputs the source code that needs to be patched into the game to include the building.
