Doing sweet wines it is best to allow them to complete fermantation first, it is extermley hard to stop a ferment in progress. Sorbate will only stop the yeast from multipling and most wine yeasts are tolerant of normal levels of sulfite. So to sweeten it is, wait unil clear either through waiting and racking, or usage of fining agents, can sterile filter if so desired now. Adding recomended dosage of sulfite to prevent MLF (50 ppm) and sorbate 1/2 tsp gallon, add sugar through taste test to proper amount, how much sugar for taste will depend on acid level and tannin content and personal preferance, wait 2 to 3 weeks checking gravity to make sure there is no signs of fermentation or you will risk bottle bombs and then bottle.