To get the FAA CFI it requires you to do the CPL & IR Exams and tests as well before you can get a CFI rating. Not it is not that easy to be honest and not worth much if you are not going to use it. With your JAA rating you can instruct for equivalent FAA licenses & ratings (as long as you are qualified on your JAA ratings to do it e.g IR & ME CRI if it applies to the FAA training being done) except for the last 3 hours which must be done by a qualified FAA CFI.