CirrusF,
If you fulfil the following hour requirements, you might get a FAA ATPL straight away, even if only hold a JAA CPL.
(a) A person who is applying for an airline transport pilot certificate with an airplane category and class rating must have at least 1,500 hours of total time as a pilot that includes at least: (1) 500 hours of cross-country flight time.
(2) 100 hours of night flight time.
(3) 75 hours of instrument flight time, in actual or simulated instrument conditions:
(i) An applicant may not receive credit for more than a total of 25 hours of simulated instrument time in a flight simulator or flight training device.
(ii) A maximum of 50 hours of training in a flight simulator or flight training device may be credited toward the instrument flight time requirements of paragraph (a)(3) if the training was accomplished in a course conducted by a training center certificated under part 142.
(4) 250 hours of flight time in an airplane as a pilot in command, or as second in command performing the duties and functions of a pilot in command while under the supervision of a pilot in command or any combination thereof, which includes at least:
(i) 100 hours of cross-country flight time ; and
(ii) 25 hours of night flight time.
(5) Not more than 100 hours of the total aeronautical experience requirements of paragraph (a) of this section may be obtained in a flight simulator or flight training device that represents an airplane, provided this aeronautical experience was obtained in an approved course conducted by a training center certificated under part 142.