What kind of game is it? what sort of levels are you interested in?

Do you have textures already? I have used apps like worldcraft to make quake maps, but it's been a while.. I used to make maps for the Torque engine.

It does seem a little old hat though, CSG levels do tend to look very boxy and it's very hard to make certain shapes.

PM me if you don't want to discuss everything in a public forum, although the more people you can get interested in helping out, the faster things will get done..