Hm, it appears minisphere doesn't implement the resizing functions! Were those Sphere 1.6? I admit I didn't take a very close look at what was new in 1.6 during initial development. Those would likely be pretty easy to add, it's surprising I never thought of it.
If you do use DaVinces advice, prepare to write game logic to set the tiles for the newly grown layers, because I'm pretty sure it'd just be filled with the tile at index 0. So you're almost the way there to creating tiles for a custom map engine, expect now you have the power of other features of sphere's map engine.