As always this is the way I understand it, but someone more knowledgable than myself (not difficult) may correct me.
Once you have acquired your FAA CPL(A) you can do an add on licence to get the FAA CPL(H), then to covert that to the JAA CPL(H) you will either have to sit at least the 9 JAA CPL exams (or 14 ATPL) and pass the JAA CPL(H) flight skills at test.
OR
After your FAA licences are completed you can sit the exams and complete a JAA CPL(H) modular course.
Either way for the JAA 185 hours and the exams are a required, the exams tend to be the thing that upsets most people and generally take up a lot of time. You do get some credit from the FAA for FW hours but the JAA aren't so accommodating.
Hope that helps?
OOTD's