How do I do that, and separate the bsp from the whole level.

They're already seperated. If you rip a campaign map (with HEK), you can see in the scenario tag that it refers to several bsps.
To convert it:
It's possible to preserve all the scenery and vehicle placements, and you could convert the sp weapon placements to mp weapon placements, ai starting locations to player spawnpoints, etc., if you want to get into it. Guerilla's import/export functions help with this.
Or you could start with a fresh multiplayer scenario, and pick the bsp and a sky, etc.
You could also, if you want to, rip an obj out of the bsp, and import the obj into gmax. Then you can get specific about seperating it.

Edited by bobbysoon on Jan 1, 2008 at 07:23 PM

How do I convert an obj file to a gmax one? Theghost's obj to gmax only gives instructions about loading gbxmodels in gmax.

bsp tag converter (pick one)
http://hce.halomaps.org/index.cfm?search=bsp+converter&B1=Search

objimport.ms
http://files.filefront.com/importobjzip/;6442433;/fileinfo.html/1/1