All that is required,
AFAIK, for a JAA PPL tailwheel endorsement is differences training from a "suitably qualified instructor". I don't think the regulations specify that this has to be a JAA instructor. So if an FAA tailwheel instructor signs your logbook with words to the effect that you've had your differences training and are qualified to a suitable standard, you should be fine. But as always LASORS is your true friend.