[Suicide Junkie]

Actually, its probably easier.
You could add Volcanic and Water surface types, for example. Asteroids would be just one more option.

Hrm...
I suppose, if you really wanted to go nuts with it, you could clear all of the atmosphere types, and add a whole lot of surface types...
Types such as:
- "Rock - Oxygen"
- "Rock - Hydrogen"
and so on...

That would allow you to limit colonization by "atmosphere" (which is now a surface type), although it would also prevent you from modding in atmosphere converters.