Hi there - I was in a similar position to you. I had an FAA PPL but I also had a lapsed CAA PPL. I made sure that when I did my FAA PPL, I flew an extra few hours so that I had done all the flying necessary to get a JAA night rating - you need some cross country time (2 hours?) and some solo (one hour?) from memory. I am pretty sure your hours will count, even though they were a few years ago.
I was able to get the CAA to give me a night rating on the basis of my US flying (in fact I had no night hours in the UK and all my US flying was under FAA rules) but the difference is that I used my current FAA SEP rating to revalidate my CAA one had the night rating attached to it. If you don't have a CAA or JAA PPL you won't be able to get a night rating as you need a CAA/JAA licence to attach it to.
I did that to avoid spending more money on additional night training when doing my CPL, but it didn't work out that much cheaper when you include the "administration" costs applied by the CAA!!
Hope that helps???