1. You will have to get either a 61.75 (based on) private or go the full route and get a regular FAA private certificate.
2. If you get the 61.75 you then complete either the foreign pilot instrument written or the regular instrument written. The instrument rating is added to the based on certificate and you are good to go as long as the JAA license is good.
3. If you get a stand alone FAA private, you would then complete the regular FAA instrument written, get the required training and do a regular checkride.