I'm no expert here...but assuming you hold a foreign ATPL and meet the experience requirements for the FAA ATPL
1. Pass the FAA Class 1 medical (there are provisions for recognizing a foreign medical as equivalent)
2. Pass the written knowledge test for the FAA ATP-H
3. Apply for an FAA PPL-H based on your foreign license (you need this license so you can do any necessary flying training, and flight test)
4. Pass the ATP flight test with a Designated Airmen Examiner (this test need not be done in an AW139, you can do it in a suitably equipped R22 if you want!)
Once you pass all the above they should put the AW139 on your FAA license if it's on your foreign license.
I think that's the process, but I could be wrong!
Salient point here is there's no bypassing the requirement for a written and flight test I'm afraid, and that is usually done on an N-registered helicopter.