A friend did this a couple of years ago. Much depends on whether you want an FAA IR as well; it is rather handy to have that privilege!! If so, the easiest route appeared to be a conversion from a JAA ATPL to an FAA single crew ATPL, which IIRC was just the foreign pilot exam. Also, an ATP doesn't need an instructor signoff to do this; you can just turn up and sit the exam.