Well, here's where is gets interesting. You can try table sugar. The only problem is that you might not know what that was made from. Cane, beets, corn, etc. So, each dfferent sugar may turn our different.
Wine making stores sell products to sweeten wines. They too contain different sweeteners. One popular product is glycerin. Glycerin is used in many commercial wineries. It is sweet, but is more of a buffer.
Sit down with your chosen sweetening products - let's say glycerin and sugar. Start out with six partial glasses of wine. Add small amounts to a glass untill you get what you are looking for. Keep adding, drop by drop, and you will find you can "dial" the flavor in better that way.
Don't forget to stabilize after you sweeten.