How to convert an EASA CPL(H) to FAA CPL Rotorcraft With a Helicopter Class Rating?
I just saw somebody try to do this. They had an EASA CPL(H) and a FAA PPL(H). The DPE needed to have a letter, sent from EASA to the FAA in Omaha, Verified, and then a copy sent to the pilot. This letter is EASA confirming the logbook entries attained in Europe were legitimate. This letter is only valid for six months. Despite getting the necessary flight training from a FAA certified CFI, as per the appropriate requirements, without this letter a DPE can’t administer a practical exam. 

You need to pass the FAA practical to get any FAA certificate. Written test requirements vary.