Good no one is interested in it, as it proved to be more complicated than I thought.
The wall:
1st way - importing separate 3d model as that back wall, with its own texture is piece of cake, but in game its not visible, I mean its there but is covered in black, as in stock game with no wall there; there is no such problem when editing whole set of walls (2nd way), but you can see a result in my previous post; but that second way of doing it looks like a job for a pro, not a noob like me

Last thing I could try is to do and import ambient occlusion map; I think it is it - those textures with green icon in s3d - that cant be exported, but can be imported... anyone knows anything specific about those?
Another issue is camera; having that wall would be pointless without free cam in cpt room; I "unlocked" the cam to be able to look behind (its nor free cam yet), but noticed those lights coming out of windows desappearing after a while (and I double checked it and know its camera settings issue, doesn't happen with untouched cameras.dat).
So, looks like its more trouble than its worth.