imports a frame?
don’t understand that…
I too think you’re making a great job for making Blender available for us amateur game artists that collaborate with several game engines…
The bsp export would be really interesting… but please do it so it’s not based in ID tools, so that the blender made bsps can be used too in comercial games…That’d rock. 
hmmm…anyway…that even I dunno if would be possible…hmm…
the fact is , main point or advantage of a bsp is lightmaps (baked lights and shadows on an apart bitmap, whose UVs are in a second uv channel) , collision info , and visibility portals, which also is essential info to port to any game.
Nice thing is that a BSP has it all packed inside and many game engines import it.
Bad thing is an old thing, and has its limits, but anyhow, a quick, free and easy way to do all…
Multitexturing and several uv channels are being worked out now or planned as I see in Blender.org. Once that appears, then that part (lightmaps) would be possible in Blender… Even if only done with plain soft shadows and no radiosity.
The thing is a void id tools. So, id compilers. you need to compile to BSP, I dunno if you could add BSP compiling functions into Blender.
But anyway, I must tell you : there are already two ways to produce free for comercial BSPs, one for q3 BSPs, and anothe rfor Halflife BSPs.
Quake 3 bsps compiler free for comercial projects (source code given):
A WIP, not ended.
http://map3bspc.sourceforge.net/
halflife BSP compiler free for comercial projects
http://www.valve-erc.com/resources/%3Fpage%3Dzhlt&e=747
These options would be in case what you do is plainly output a HL or Q3 *.map file, so that then, once can pick one of these compilers and output a BSP. Dunno what is harder to do.
Map editor that can be used for comercial projects (GTKradiant or qeradiant, can’t, as stated in their pages.)
QUARK
www.planetquake.com/quark
As you see, there’s already a path for doing this fully. Just in case you want to avoid some work or all of it.
Still, Quark and all thos eeditors are much more limited in geometry editing, but is that I suspect also BSP format is limited in type of geometry it can have.
At last, there’s a freeware (not open source) BSP map editor and compiler. called Getic (yet to know if is quake3 bsps, hl bsps, or just personal form of bsp):
http://getic.njoydeco.com/
Anyway, I don’t think it would be easy, and quite a hard job…
But anyway, I hope the info may come handy…