If you're only going to do hour building in the US, then the easiest way of doing this is by getting a "piggyback" PPL. This is an FAA PPL whose validity is dependent on the validity of the underlying JAA license. Search the forum archives (if they let you) for the word piggyback and there's a lot of info.
This is the official place to start:
http://www.faa.gov/licenses_certific..._verification/
If your intention is to add ratings to your FAA PPL (like ME, IR) it's best to do this on a standalone FAA license, which means doing the PPL theory exam and the flight test all over again, but this time FAA-style. This avoids building a house of cards.