With some yeasts you may do, but do not add more than the recipe or manufacturer recommends as yeast varieties are selected for different properties. A yeast variety which can ferment rapidly, may not be very resistant to alcohol. If this is the case, adding too much sugar will stress the yeast and may result in unfermented sugar remaining in the wash. This can often cause the wash to foam up during distillation. Most spirit yeasts have a recipe printed on them, with the amount of sugar and there ideal fermentation temperature. Select the yeast that best suits your needs.
Generally yeasts will have only two options, either fermenting 6kg of white sugar (7kg when using dextrose), or they will ferment 8kg of white sugar (9kg when using dextrose). If the yeast gives the option to ferment either 6kg OR 8kg of white sugar, it is ok to add the extra 2kgs, however usually it will state that a faster and cleaner ferment will be accomplished using the lesser amount of sugar. This is due to the extra work and stress put on the yeast when adding the additional fermentables.