You are talking about Chaptalization (adding sugar)
Here are some calculators you will find handy
I have been adding table sugar to my wine when I want to up the alcohol. I heat water and add the sugar and stir till its dissolved and ad it to the must. I usually add 1 to 2 pounds sugar to 2 to 4 cups of water and add that to the must. I was reading that a better way is to use Grape Concentrate and sugar. The Winexpert Grape Concentrate is 68° Brix.
Red Grape Concentrate
White Grape Concentrate
To bump up a Winexpert Island Mist kit you would use 1 Grape Concentrate pack and 1 pond of sugar.
You decide how much alcohol you want and add the appropriate amount of sugar.
Hope this helps