What do is I cut the limb at the joint (hacksaw - or Dremel if the material is softer). Then I sand the areas to be joined with heavy grit sandpaper. I drill holes into the ends of each piece of the limb where it's to be joined. I shove pliable wire into the holes, and sort of lace the limb into the new position. Sometimes it needs extra temporary support, like a sling, or rest it on something high, if it's an arm or whatever. Then I lay fiberglass over the lacing wire and let that set up. Usually a couple of layers. When it sets up, it's in a new position.
I'm actually getting ready to do this on one of my new mannequins soon.
I also make the fingers poseable so you can get them to hold weapons. Someone suggested using zip ties (i guess at the wrist) to secure the weapon to the hand even more securely. Sounds like a good idea to me.