Good question by Eric, but I have done the same. You need the 3D max tools from the rFactor web site in the Developers area. They now support later versions of Max up to 2010.
It took me 700 hours to do Lakeside from scratch, as most of that was learning Max, photoshop, BTB etc.
You need the GMT importer plugin, no idea where to get that now that RSC is dead. It is around some place.
The ONLY way to import to Max is to export the track you have to rFactor as normal. Then use the GMT plugin to import to Max. All your image settings will be wrong now and must be redone. You then use the ISI tools to export to your game/track location.
The reason I did this is because I started Lakeside before X-Packs and did the basic terrain and track and wall locations in BTB. All my buildings are done in Max and are way superior to Sketchup. Sketchup is OK but you can not get the detail other than a program like Max or similar. Locating objects is easier too and textures are better handled as well. However, trees are way easier in BTB. Way better....
Having said that, my Top Gear Test Track will be made in BTB as much as possible with the awsome XPacks available now. I think only the buildings will be done in Max and maybe the terrain height, but that will be it.
Woops, almost forgot. Joining tracks in Max is a little better as you can cut and splice poloys and also weld verticies etc.
Mike