One would think the original Laser Scan of Sebring would have included the environs behind the regular pits that were used by WEC for their race & and we do know the extended road past Turn 5 is already there in the current build. What would it take to expand the Sebring offering?
I think the '98 12-hr circuit would just be an AIW (provided there aren't any barriers there). Any motivated modder can do that. Other versions would be more involved and require S397.
pretty sure historic versions would also involve a license fee for the studio besides the modeling work, so it wouldn't be an expansion but rather an all new dlc item (except for the 98 version if it only requires an alternative aiw like emery says)