Hi Andy,
Exactly what I did last year, obtain an FAA CPL/IR that is.
To convert it you need to do the following:
1) Study for the 14 JAR-FCL ATPL Written Exams at an approved training organisation.
2) Pass all of those dreaded 14 JAR-FCL ATPL Exams.
3) Training as required to pass the CPL Skills Test
4) 15 Hours JAA IR course and skills test.
The above assumes you have a FAA CPL/IR Single and Multi ratings.
That'll give you a JAA CPL/IR with ATPL Subjects passed.
All of the requirements are detailed in LASORS.
I'm doing the ATPL Writtens at the moment.
Best of luck,
Charlie Zulu.