Hi ArcherII,
If you have an FAA Commercial and Instrument (Multi) then you'll need to go through the following conversion process to gain a JAR-FCL licence.
1) ATPL Ground Examinations via aproved course.
2) Enough Training at discretion of Chief Flying Instructor to get you through the JAA CPL Skills Test. Average around 10 hours.
3) A minimum of 15 hours JAA IR training which I believe 5 hours can be on an approved simulator.
That'll mean you'll have both a full JAA CPL/IR with ATPL Writtens out of the way *AND* an FAA CPL/IR Single Multi to go with it.
If you also have a FAA CFI or CFII rating then you'll be able to convert that to an JAA FI(R) rating for around 15 hours flight training if I remember correctly.
Having both a JAA and FAA certificates will hopefully open up a lot of different options, especially in the corporate market. Why else am I going exactly the same route?
All the best,
Charlie Zulu.