Actually, someone did it. But the source engine couldn't handle its awesomeness so they coded their own engine. It is called Natural Selection 2.
jk...
Personally, I wouldn't port tremulous to any other engine. Even If you did, what would it add to tremulous. Too much work for a few extra features.
If you are thinking about a commercial game you should reconsider it. "Aliens vs Blahs" is an overused theme; you'd need to find some unique elements.
Anyway I don't think you are serious.(If you were you'd have posted some sketches and/or screenshots)