No problem adding solid sugar. Bartenders use a syrup because it instantly dissolves. Since we usually sweeten and let the wine sit for a some time before bottling, there's no rush for the sugar to dissolve. I add it, give it a good careful stir and then come back a week later and stir it one more time. Or just wait to rack and bottle at a later date. The sugar gets plenty mixed up and dissolved.
Another alternative is to take a portion of your wine and us that to gently heat and dissolve your sugar (2:1 sugar to wine), then add that syrup back. I wouldn't heat it too much. That way you don't dilute your wine by adding water.