Hi !
Well, some things are not yet possible in CityEngine, but CE is also not specialized in manual modeling, but is rather a procedural modeling tool. Nevertheless we're constantly increasing the stability and usability of the tools, including those shape creation tools. It can be that CE 2013 will solve this specific situation better, there was a lot of effort put into those modeling tools lately.
Depending on the complexity of construction you need on those roof geometries, you may at the moment be better off with a CAD application, sadly.
So at the moment, I think, subdividing may be your only chance on this in CE currently.
Matt