I 99% sure you can't convert a FAA CFI to a JAA FI like you can a FAA CPL to a JAA CPL.
You'd need to take the full FI course. I'm not sure if you'd be an FI(R) though? You'd have say 500+hrs of instructing CFI / MEI / CFII under your belt, and no doubt the experience you'd have built up in the states will help you through the JAA FA system.
Good luck