A JAA ATPL as held by airline crew often (but does not necessarily) includes only a multi-crew IR and IMC-rating privileges associated with Private flying in GA.
On that basis, I suspect that the FAA would require the usual three hours instruction, flight test and ground exam for an unencumbered IR to be added to the FAA PPL.
On the other hand, they are pragmatic types in this kind of area. You could do worse than to ask and see.
2D