Of course you can add sugar. As long you have only a small stock of spirits at home, it's ok IMO. It's even ok to use sugar for 18% and turbo yeast IMO. Not those 48h ones. There are better turbo yeasts, simply an alcohol tolerant wine yeast and the needed nutrients for a slow but complete fermentation up to 18%, the Prestige fruit schnapps yeast for example. Back on topic:
Pushing a 6% mash with sugar to 12% will not half the aroma. But it will have noticeable less aroma. So when you come to a point where the amount of product isn't such important anymore, I would stop adding sugar.