Tracks are not possible, due to needing the developers SDK to create some of the files needed. Been this way for all of the more recent SMS games (NFS Shift, S2U, TDFRL and pCars)Fantastic work, thanks to all the team for all the fantastic cars you guys have given us. Will tracks ever be possible?
[side note] I was only able to add Xbox DLC tracks to NFS Shift, by byte flipping the physics mesh file and converting the SGB file via a tool made by a coder. Well...and literally thousands of other hex edits.