Apart from the "Why? The stuff's horrid" thing, doesn't it have preservatives?
With so many tasty juices around that don't have preservatives, I don't quite get why people bother with the extra hassle.
But anyway....
Boil off the preservatives, add some sort of tannin (grape juice, raisins, powdered tannin, etc), calculate the sugars to see how much you need to add, check the generic WOW recipe.