Think you will struggle to find an adapter to take a conventional rear derailleur unless you find a frame builder..Perhaps a dual drive hub may be cheaper or even a 8 or 11 hub gear may be the way to go..Sell your old P8 and buy a new one..The ones currently on sale in Thailand have a braze on for a front derailleur and a conventional gear hangar at the rear..presumably they will have in the USA..