What do you mean by "adding the underlying code"?
There are a whole lot of changes that need to be made to the DLL before we can implement multi-maps. For instance, the game needs to store three dimensions of plot data (the third dimension will determine what map it's on) in the memory, and needs to recognize each z-level as an independent map gameobject. Also, there will need to be work to get the AI to understand the existence of multiple maps, as well as code added for interactions between maps, even before we can have Multi-Maps. The trigger for actually getting Multiple Maps would be AIAndy or DH writing a mapscript to generate a Lunar and Solar System map, the first two one would encounter.