does anyone know whether you can add ratings to a FAA PPL issued on the basis of a UK licence?
Yes, definitely.
However, I would advise (if you are going to the USA anyway) picking up a
standalone FAA PPL while you are flying there. This gives you independence from the European licensing scene, and avoids building a house of cards.
i believe a rating like this would be transferable to your UK licence when you return
I very much doubt it; maybe somebody here knows? Europe accepts almost no ICAO licenses/ratings for conversion purposes.