Unless you have a refiner, you can't add sugar to it without it being grainy. Sugar simply won't dissolve in chocolate.
There are many things you can do with this chocolate, two of them being:
1. You can use this chocolate for baking.
2. You can blend this chocolate with other chocolate to create different intensities. It would be helpful to you if you knew the actual fat % vs cocoa solids %. However, you could definitely take a very sweet milk chocolate and add this to it to intensify it and reduce the sweetness.
Hope this helps.