Generic frozen apple juice often does not have sorbate or other anti-yeast preservatives in it, so most any brand should be good. Supposedly one frozen thing makes about 48 oz. of juice, so depending on how far to dryness the cider/apfelwein that you want to sweeten went, you could probably be good with 1 frozen can per gallon.
Sweetening is pretty much always "to taste". If you want to carb it, then you need to do some math.