As Gordy pasted,
(3) Hold either a foreign airline transport pilot or foreign commercial pilot license and an instrument rating, without limitations issued by a contracting State to the Convention on International Civil Aviation.
if you have a JAR CPL and IR you can just take the ATP written exam. As for the period of validity you'll have to check. I have a JAR ATPL(H) IR and took the FAA ATP written in UK, then 3 months later took the ATP flight test in USA.