I'm not of much help with attaching a new hipbelt to an old pack, but I know Kondos will do it for you on one of their packs.  Seems like it costs an additional $13 for a new pack.  You may want to call them.

Otherwise, the design is pretty straightforward.  They use an additional piece of pack cloth and stitch it horizontally to create a sleeve for the hipbelt to slide through.  If you need a picture, PM me.

I did call Kondo's today and was pleasantly surprised on how reasonable the price was. When I explained that packs seemed to slip after cinching them tight on your back,  he said make a note of that and he would look at it too. So far I am very impressed with service.

Just a quick follow up, contacted Kondos they added the hip belts, and shoulder straps.

I also mentioned that we had problems with the packs "slipping" (When the pack was hoisted on to your back the packs tended to slip and did not sit ideally) So they changed out the metal buckles and staps with a different style that would more match our style.  They finally patched a few small holes that where developing. (I hadn't even noticed them)

They did this in less than a month and it cost less than it would have I had purchased waist belts and installed them myself. All in all excellent customer service. In fact, far more that I would have reasonablly expected from them.

Glad the folks at Kondos treated you well.  They've always been great to work with.