Why do you not want the yeast to ferment the additional fruit sugars? I think that the yeast byproducts of fermenting the fruit sugars would be what you want in a fruit beer. Unless you are looking to retain sweetness from the fruit, but cranberries don't have a ton of sweetness in the first place. If that is the route you are looking to go, then why not try adding some actual cranberry juice (without preservatives of course) to give you that fruit punch? Or better yet, try adding the fruit and the fruit juice.