Don't know, didn't got answer when I asked the last time. So the only way to get it ingame, is to create a 3d model again. I have my old material still, but it was never really detailed, as I just wanted to make the basic as model and the rest via painting tools, etc. If I will try my luck again on the cathedral, I will redo the model. By now, I am a bit more skilled in creating new Fallout art, than when I started this here. :>
I don't know how to add new stuff to Fallout 1 or if it works at all. My plans for the cathedral have been involving FOnline: 2238. But as I've destroyed the cathedral ingame now, to make the setting more in line with the original canon, I am not really in the need of this now. The whole plan has been to merge the inside and outside maps, to make one big thing, which would have been absolute epic.
The FOnline engine supports bigger tiles, so I would just have to create several 500x500px tiles and then it would have been done.