In a nutshell, to convert an FAA CPL/IR into a JAA CPL/IR requires the following:
All JAA ground exams to be taken; either CPL and IR theory or ATPL theory - cost around £2k including exams fees.
The CPL conversion is "hours required" in order to pass the JAA skill test. I have heard that some FAA CPLs can take almost the full 30 hours anyway due to differences in airspace, RT, rules, airspace etc.
The IR conversion is a minimum of 15 hours (and has to be completed in JAA airspace) and a twin engine type rating must also be ontained. Some of this time can be obtained in the sim but some must be in the aircraft at around £1,200 an hour. Initial type rating on a twin is 8 hours.
Cheers
Whirls