No.
Well, T3DGM uses little sections, probably so that it can have some sort of detail hiding code. I'm not too sure about collision detection, I reckon it uses an internal box collision routine - so your pretty much stuck with the prefab levels. There is the maze level editor which I nagged and nagged for while beta testing, but it's a bit limited.
Not great news. You could change the level component models, but not the collision data.
For the future...
There's is a new game creation package on the horizon from the TGC team, FPS creator - which is looking pretty damn cool and will have it's own level editors and lightmapper. Might be more what your after, T3DGM is written in DarkBASIC v1.09 - which is about 3 years old now - FPS creator is being written in C++ using DBPro .dlls so should be capable of so much more.
Van-B
Next time he runs past, GRAB HIM!