+1 for Bilenky for retrofits and any other work. I also live in the Philadelphia area, and they done work on a few of my bikes, from adding rack brazeons to two bikes, to kickstand plates for a few others. Always a good job and reasonably priced.

Best to read the info they have on their site about retrofits: Bilenky Cycle Works

If you want your bike retrofitted, whether at Bilenky or elsewhere, better get moving on it ASAP as they usually have a backlog.