I don't think you necessarily need to get your type ratings on your U.S. certificate. If you can convert your JAA license to a US ATP MEL that should suffice to apply to the airlines along with a class I FAA medical. The fact that your JAA Type is not on your FAA Airman Certificate in no way negates the experience you have on type and documented in your log book. Why spend $10,000 on a type rating when you can get the ATP conversion MEL on a Seneca or something similar at a flying school for a lot less. Airlines will train you on type and give you a type if its part of their training protocol.