I'd recommend a standalone, not piggyback, FAA PPL. Otherwise, you are building a house of cards.
Since the checkride must be done in the USA, may as well do the lot there. From a JAA PPL position, you need to learn the FAA ground school, and do a few hours' flight training and the checkride. Then you have an FAA PPL which is valid for life.
Some details of the piggyback process are at
www.pplir.org - an excellent site for pilots wishing to achieve an IR, or an IMC Rating.