Hi
Having looked into this myself here's what I can offer.
If you have a JAA full ATPL issued then, you have to do the FAA Written ATP exam which can be done at Flight Safety Farnborough. Once you have a pass with that, then you can get the FAA ATP added when you do either your initial Type rating, or when you do the recurrency training on the Sim in the US. It really is as simple as that if you have a full JAA ATPL !!!!
Now if you don't, well things get a wee bit more complicated.
You have to convert both your CPL and IR seperately and that means two seperate flight tests (in the aircraft) with a FAA examiner. You have to also do both the writtens, FAA Commercial Pilot and FAA Instrument Pilot as well.
This can be done in this country with the writtens again at Flight Safety Farnborough and the flying training at an FAA approved school. There are a number of training centre's that will do this, one in particular is Willow Air in Southend. They train for the FAA tickets and fly an examiner over from the states every 8 - 10 weeks or so I believe. Training is as required as you already hold a CPL/IR, so you wont have to do the full course. As far as costs go............. I have been quoted anything from between £4000 - 8000 GBP Also, I am not too sure of the implications and timescale, in getting the license issued from a foreign state, worth looking into!!!!
Obviously, your other option is to go to the states and convert. It may work out cheaper, but you would have all that unfamiliar airspace and RT to contend with.
Good Luck.