Hiya.

Basically, you have to re-build it by hand. As far as I know, there isn't any way to "extract the .map from the .bsp". QuArK will let you look at a .bsp as a map, but you can't "resave" it as .map. The .bsp, from what I gather, is just a list of what surfaces are viewable from a particular location...there isn't really any "brush" information. Sorry!