Hi beer monster,
In most recipes it is usual for the sugar to be added at the start of fermentation. By adding the sugar 6 days after the other ingredients you run the risk of the yeast all being used up by the sugar in the juice & not able to cope with a sudden introduction of more sugar. The golden rule is not to add too much sugar initially, no more than 1 kilo of sugar per gallon in the mix. If you are making a dessert style wine which needs to have a higher alcohol content , then you should dissolve any extra sugar in water and add to the fermenting wine in small doses as fermentation progresses.
Adrian.