as far as i know, to conver the FAA ATP to an ICAO one, you need to do a couple of written papers. And since we all know that the FAA isnt of the highest standard, it is going to be tough. I did the ICAO ATP which was suposed to take 8 weeks in about 15 since there was so much of catching up to do after doing a FAA CPL. its not going to be easy but if you work ur butt off, you might be able to do it in about 8-10 weeks. But i am not too sure if FAA ATP is counted as an ICAO one.