I don't think the hot glue will work, as it's essentially just wax. I've heard that jewelry makers use a heavy-duty two-step epoxy for that sort of thing. You might contact a jewelry-maker's supply house or store.
I've turned screw-back earrings into post ones by cutting off the screw-backs and epoxying to it a post backing. I'm told this sort of arrangement tends not to be real secure. I've also bent the earring backing into a loop, after cutting off the screw-back, and attached a fish hook.
I buy the glue and the findings at a local beading/jewelry-making store.
I recently restrung some flapper beads I bought on ebay years ago; its old string disintegrated while I was wearing them one evening. I found directions on how to re-string and attach the bead tips on the jewelry-making page on About.com. It has lots of good clear explanatory photos. I also got some advice at the local bead store.
I've turned some old wind-up watches and watches into very cool purse charms by adding jewelry findings like jump rings and clips. And I've attached some old dress clips to the tops of purses, securing them with sewing stitches (upholstery cord) or jump rings.
This sort of thing can get very addictive so I've stopped buying old jewelry at garage sales. Still people give me old stuff and I feel compelled to do something with it.
