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!
^_^
"We've got a blind date with destiny...and it looks like she's ordered the lobster."
Use the tool MilkShape3D, but it's not freeware. You can use it for 14 days, if you haven't registered it. MilkShape can also import and export a lot of other 3DModelFiles.