What kind of metal are the charms? anything other then silver, stainless steel or regular steel will be difficult. with the mentioned metals they can be silver soldered to a back pin of some kind and instant concho. with pot metals not so easy, the pot metal will melt before the solder. you may be able of epoxy a post to the charm, but it is almost certain to pop off. you might also try to solder to charms to a post with stabrite solder, a low temp solder for things like pot metal, but would be iffy in that you still might melt the charms first, but it is probably your best and strongest option.