Not sure about converting an FAA CPL, it sounds more like doing everything over again rather than a conversion.
I would suggest get an FAA ATPL first, then convert to JAR. It's easy ( aside from the hours of study) and you're not unlikely to get a job in Europe without the ATPL flight hours anyhow.Converitng an FAA APTto JAR CPL is the way to go, and a joke in Spain.
I did my CPL flight test and IR check in Madrid a year ago. The standards there are appalling. Half the instruments in the aircraft didn’t work, so the IR check turned out being a VFR joy flight!
That was lucky as I was half pissed at the time.
I bought a bottle of orange juice mix in a grocery store and downed it for breakfast. I don’t read Spanish, but the label said something like 8% real juice—so I thought. Turned out it was 8% alcohol! I keeped my month shut and just to the flight as I had my ticket out of Madrid booked.
The CPL test, IFR check and training took a total of 14.4 hours over 3 days, and cost me 4000euro for the school and a few hundred euro for the examiner.