ISI has a tool for that where you drive around the track & create the main path & corridors, Fast Path, pit lane path, pit positions.....the whole lot with a few extra features that are not available to the BTB AIW tool...
You can download it from rFactor central or there's links available in one of the threads within the BTB section here at RD.
Sorry, Im not sure where exactly, you'll need to look around for it as Im just going by memory...
I hope this is of some use to you.