I first want to clarify - RBR needs only Centerline. I wrote about Driveline - mismatch - sorry!
I don't know what you mean exactly? If rF or GTR2 tracks can work for RBR this way? I think they can.
My idea is to add new track just for attaching (Appending) the "open" drivelines - I recommend to delete that dummy track after you have new centerline. It's not necessary to add invisible and non-driveable objects to your track. They would be not rendered, but still calculated by the machine. A waste, if you ask me.
For RBR you have to make laps manually - in BTB.
One other idea is to get unlimited laps in RBR (maybe for testing?) - attach drivelines to closed circuit, switch to Positions on that window and set all Start, End and Finish gates to 0,0 meters. Sometime you will get "Wrong Way" message in game, but you can drive forever this way
Remember - to replace "closed" driveline you have to clear all drivelines first, then attach new to open track.