Once you create the prop, you can save it as an actor. Then, when you bring it into a scene, click on the weapon and choose "Conform To" from the Figure pull-down menu.

I looked all over for a Ski cap for M3 and couldn't find one so I made it in Rhino, then after texturing, made it conform to my Michael morph of al Zarqawi's head. First attempt! Honestly it scared me how easily it came together... (lol)

Yank My Doodle, It's a Dandy!