Sugar is used by some wine producers (it's called chaptalisation). Primarily to increase the alcohol content - it gives the yeast more to work with, but it usually results in a sweeter wine as well - especially if you stop the fermentation before it's complete.
I had a quick look for methods of stopping fermentation (I thought you just added grape alcohol) and came across this regarding acidic wines...
Cold Stabilization (from
http://winemaking.jackkeller.net/finishin.asp)
Sometimes called chillproofing, cold stabilization is a good method of smoothing out an overly acidic wine. It also aids stabilization and oaking and helps refine wines that possess a "bite."
When fermentation is complete, the wine is racked into a clean secondary and moved into a cold garage, workroom or storage shed, or into a refrigerator, and allowed to age for several weeks to months at temperatures just above freezing. For overly acidic wines, a portion of the tartaric acid will precipitate out as crystals. The wine is later removed from cold storage, racked off the crystals and bottled.
A fermenting wine brought down to very low temperatures and held there for several weeks will stabilize. Some yeasts can tolerate cold better than others, but as the wine approaches its freezing temperature almost all yeasts will expire. The freezing temperature of wine depends on its alcohol content. A 12% alcohol wine will not freeze until chilled below 25 degrees F., so bringing it down to 32 or 30 or 28 degrees F. for two weeks will not endanger it but will kill off the yeast. However, do not put your wine in a freezer and leave it there as it will undoubtedly freeze solid and break the carboy.
An old refrigerator with shelves removed is perfect for year-round cold stabilization and doesn't take up too much room in the garage, basement or workroom. Adjust the temperature to its lowest setting. If the refrigerator has an internal freezer compartment, remove the divider between the freezer and refrigeration compartments so the temperature in the refrigeration compartment can drop even lower. Your goal is to get the temperature down to 40 degrees F. or lower. Leave one or two shelves in if you make your wine in gallon batches, but never ever try to put a 5-gallon carboy on a refrigerator shelf.
If you do not have an extra refrigerator available, there is a field expedient method of cold stabilization. First treat the wine with potassium sorbate and potassium metabisulfite to prevent further yeast reproduction. Then put the carboy in a plastic garbage can just big enough to leave 2-3 inches of air all around the carboy. Fill that space with ice cubes or crushed ice and sprinkle about ¼ to ½ pound of salt on the ice. Stir the ice a bit with a wooden spoon and then let nature take its course. You may get some ice formation on the insides of the carboy, but it shouldn't grow very thick. It is prudent to mark the wine level on the neck of the carboy before starting. If the wine level rises more than ¼ inch (5 mm), remove the carboy until the inside ice melts and then put it back inside the ice slurry. Add ice and salt as necessary. When the slurry loses its frigidness in a day or two, dump out the water and repeat. You'll have to keep it ice cold for at least two weeks, but this does work.
