The FAA PPL does not have a night rating as such. The FAA PPLs are automatically allowed to fly at night as some of the PPL training includes night training.
I suppose it is at the descretion of whoever carrys out the conversion whether they will also give you a Night qualification on the JAA PPL.
Doesn't matter however, as you could use your privileges of the FAA "night" portion to fly at night anyway, so long as the FAA ticket is still valid.