A few thoughts.
The yeast will stop fermenting when there is no food or there is a hostile environment (high alcohol, too hot, too cold, pH, competition and others). Your must is 3.5% alcohol, so it is not a hostile environment, so chances are it ran out of nutrient and I'm thinking the pH is really high. What I would do is add tartaric acid to get the pH around 3.3, add the proper amount of nutrient and try restarting.
Say this sugar wine did ferment to completion. The result would be a pretty tasteless (at best) 87% water, 13% alcohol solution. Topping with this will still water the taste of the other wine down. I would look for a similar or complementary commercial wine to top with.
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