Do I add sugar to fermenting juice?

Ive been letting about half gallon of juice ferment for about a week, trying to make some homemade alcohol. I recently learned that I can make the drink more alcoholic if I add more sugar to the juice. Now, if I take the seal of my container and add more sugar after its been fermenting for a week, would that mess up the fermentation process? I know its childish, and I'd have better luck just asking one of my dealer friends for some Gentleman Jack, but I'm not even really doing it for the alcohol, its just fun for me to make it myself.