Messing with Mob Equipment in Vanilla
To make a mob able to pick up gear:
/entitydata @e[name=ENTITYNAME] {CanPickUpLoot:1b}
If the mob already has something in the slot you want to fill, use /replaceitem to get rid of it:
/replaceitem entity @e[name=ENTITYNAME] slot.armor.head air
Then teleport the item to the mob:
/tp @e[type=Item] @e[name=ENTITYNAME]
You'll probably want to put more arguments into the item selector to only select the item you want.