my 2 cents.... .. .There is no actual ICAO to JAA "conversion" unless the country that issued the licence has a BASA (basic aviation safety agreement ?) signed with the JAA. The FAA hasn not-so far.. .. .However, holders of an ICAO licence can be credited with their previous experience towards a JAA licence. (JAR-FCL 1.016). .. .In my case-I hold an FAA ATP. I was told I had to pass the full JAA ATPL written (distance learning for example from Horizon swiss flight academy)and a checkride. Which will get me a JAA CPL-IR with frozen ATPL. For the full ATPL one needs 500 h of multi crew time.. .. .Don't know the procedure for anything lower than an ATP though, talk to an FTO e.g. Horizon !