The ISD was never meant to enter the Atmosphere, It's not the same ship as the RAS. Unlike the Republic, the Imperials decided to use Landing craft, dropships and things like that instead of landing the whole cruiser. (Which would probably be more effecient anyway, considering it's much more effective in combat while in space. It might even be vulnerable if it landed.) Still that's looking at the subject from a strictly purist point of view. And it is a game, messing stuff up is what modding is all about (well kind of). It'd be kind of cool, but it'd be some task to try to make it.