I don't think you will be able to get an FAA PPL issued on the basis on an expired JAA PPL. But i'm not entirely sure. You best check with the Orlando FSDO
North Florida FSDO
Also, I would check LASORS to see how long your ATPL/CPL exam credits are valid for. You might need to resit the exams before being allowed a JAA CPL.
LASORS: LASORS 2008 | Publications | CAA
Also, you might want to look into how healthy the CFI job market is in the USA (ie not very). If the academy doesn't hire you (they don't hire everyone), then you will be amongst TONS of CFIs currently looking for a job in the USA. I know someone looking at the moment and he is constantly being told "we require 400/500 hours". Even experienced pilots are struggling to find work in the USA.