You can add one crushed campden tablet per gallon to the finished wine. There are a couple of reasons for this- one is that campden (sulfite) is a preservative/antioxidant so that will help keep a low ABV wine. Also, sorbate works far better in the presence of sulfite and you would have less chance of fermentation restarting by using it.
After that you can sweeten to taste. I normally let my wines totally clear before stabilizing with campden and sorbate as once they are totally clear they are ready to be bottled.
For sweetening without diluting with water too much, consider sweetening with fresh must (lemon and sugar and water or whatever you had in there) for the best flavor instead of a watery flavor.