Yes, ITs posisible, And I have done it.
But you need to edit the .fpi for each floor to allow CSGs.
Im not sure where I got it, and I have lost the Readme, but It is called GPS (In the FPSC SegmentBank) and there are 2 versions (Sci fi, and Medieval)
Im pretty sure I got it from
http://www.fpsfree.com/
Search Trapdoor In the download section, Theres Also some handy Ladders from xplosys (its all mostly made by him
) But I have some troubblew getting back through them with a ladder or even stairs.
And A tip if you do get, or even have the ladder, lean it on its site using ctrl+2 a little bit.
Hope I helped