As far as I can see, if you have an FAA CPL/IR then you'll be able to take the JAR-FCL CPL(A) and IR exams without the full courses of training.
However this will only entitle you to hold a JAA CPL(A)/IR.
If you would like to "upgrade" to the JAA ATPL(A) (or so called "frozen" ATPL(A)) then the JAR-FCL CPL(A) and IR exams will count for nothing. You will need to undertake and pass all 14 JAA ATPL examinations. Whether you would need to do the full 650 hour course or just "training as required", I do not know.