I agree this is the time to do it - I have made a few of these kits and almost always some sugar, although I don't try to go over 10% alcohol. If you don't get the sugar added before/during fermentation, you *could* try to restart the fermentation with sugar and yeast after it finishes, but that is risky and difficult - you could end up with extra sweet, low alcohol swill. Your only other option (and it's a poor substitute too, IMHO) would be to add everclear after it has finished fermenting or right before bottling. The Everclear will simply add 'heat' from the alcohol at first and will need several months to blend and smooth out, and I'm sure you don't want to wait that long.