First use the Tunnel Maker and apply the tunnel to the specific parts of the ride. Then, export the tunnel to a .3ds; after, you import this .3ds into Google Sketchup (Or any other 3d software).
From there, create your ground using a simple plane that goes over your desired points on the tunnel - or get creative. The texture for the ground is in the NoLimits folder on your computer.
Once that is done, export the new 3d model as a .3ds and you are ready to import into NoLimits*.
I hope this helps!
*You may need to fix the shadow problem from Sketchup*