Do you mean entity buildings like those supplied with FPSC (WW2 single room building, small bombed farm building etc), or buildings made from segments, allowing you to enter them?
If it's the former, then you might find some useful info in this thread
http://forum.thegamecreators.com/?m=forum_view&t=66065&b=24
If it's the latter then you've got an awful lot of work ahead of you
I suggest you start by download the FPSC Segment Editor (see the downloads section of the main FPSC site) and watching the video tutorials that come with it.
Good luck!