When a Jew is making wine, does adding sugar in the process make the wine non-kosher?
The wine is still kosher to drink according to all opinions. The bracha on wine that has sugar added is still borei pri hagofen. Regarding kiddush however, there is a preference to wine without sugar added. According to the Ashkenazi custom the preference extends to the fact that if a person has two wines that are equal, then it is better to make the kiddush on the wine without sugar. If, however the sugared wine is better, it is fine to make kiddush on it. The Sefardi custom is that the sugared wine if always preferred.
O:CH 272-8, 202-1