You only sweeten when the sg is stable meaning hopefully at or below 1.000 for a few days without movement. You will want to add the sulfite and then the sorbate to prevent any refermentation and then sweeten this wine with either a simple syrup made up of 2 cups of sugar to 1 cup of boiling water or you can use a wine conditioner but you still need to add the sulfite and sorbate as the wine conditioner only contains enough sorbate in there to prevent the product itself from fermenting. It is basically an inverted sugar ready to use but its much cheaper to make it yourself. I prefer to sweeten my wine before clearing the wine as it can sometimes cloud the wine up again.