You would need something like 3dmax, Zmodeler, Blender etc.
Because I don't own 3dmax, 3dsimed or Zmodeler I use Blender. I haven't done this yet but I had this thought. It's a lot of stuffing around but, you can only use what tools are available.
[ED] I just did a small project from this and it worked.
The poor man's way
Basically:-
Finish track in BTB ->
Export from BTB as .x ->
Import .x to Blender ->
Highlight and join selected meshes ->
Weld/aligh/etc ->
Highlight and separate meshes back into their relevant sections ->
Export as .3ds ->
Create xpack, add objects and adjust heights to 0 ->
Create a new project in BTB ->
Create a very small circle track with roughly about 5m inside diameter ->
Add terrain, highlight and merge inside vertices so you get a centre reference point >
In 3d window zoom in as close as you can get to the centre reference point ->
Add the xpack into the project ->
Open objects dialogue and add each peice one by one using the centre reference point to click to add each piece (the closer you get and the more prominent the centre reference point is, the better chance of having it all line up).
Set Material Name in Venue Materials dialogue (ie roada, gras, grvl) ->
Add a new track over the existing track and line it up roughly ->
Append aiw ->
Export to rfactor/rbr ->
Open .scn file, remove the extra track you added, the initial small circle track and remove the .gmt files aswell ->
If you have 3dsimed you can do any fine tuning of object positions (if they didnt line up properly in BTB)
That's about it, like I said, it's a lot of stuffing around but in my case, its the only legal way I can think of doing it
