I'm certainly not the expect on PAL/NTSC, but since no one else has replied:

At the very, very least, you'd have to export all of your finished PAL footage into a brand new project (or worse, seperate software) and re-encode and rebuild your DVD from the ground up. The worst case scenario has you investing many hours into completing this request for your client.

If I were you: do some more research on the conversion process, then make sure you have the appropriate software to even create both types. Then, and only for a significant additional charge, tell your client this can be done.

Honestly, I wouldn't consider offering this service.