faa
Hi,
I don't know much about the JAA system but the FAA does require you to hold a commercial certificate before you can get a CFI certificate.
However I believe you can work for a JAA flight school within the US with only JAA ratings however you cannot give logbook endorsements EG first solo etc... for a US registered airplane.
Conversion training is not a big deal and there are a million schools which offer cheap, quick conversion training.
Good luck
WEasil