Can I dissolve sugar first before steeping tea?
No matter when you add your sugar it will still form a nasty crust in your kettle as it will not fully dissolve without stirring. In fact brewing tea in your kettle directly is not a good idea either as it will discolor it over time and may even impart flavors depending on the material. You need to pour the water into something else to brew it in. 

As to when to add the sugar it will definitely have an impact on the tea brewing, whether that is negative or positive will depend on your personal taste, the amount of sugar added, and the kind of tea you are using. Tea brews better when water still has dissolved oxygen, and adding sugar may reduce the available oxygen. It may also interfere with the water extracting flavors from the tea, which could change the resulting flavor. You'd need to try it both ways and see. 

Try not to overbrew, 5 minutes should be enough, after that you'll start to extract more tannins.