A rule of thumb with brewing is when you brew with non-malt sugars, you want them to make up less than 1/3 of the ABV level of your beer. So with your standard refill, if you add any additional sugars, you should sub them for part of the Booster, which is already 2 cups of corn sugar. More malt would be the way to go to increase the ABV of the refill.
To brew up a beer with a little higher ABV level, one of the easiest things to do is order either a Deluxe All Malt refill or a Craft Series Refill. Then, to boost it higher, add a Booster pack to reach a higher ABV. You might also check our recipe page, where we have ABVs all over the map. The Recipe page is also a good place to look at how we use different sugars, because there is not just one way - it depends on the type of sugar, etc. So there is no blanket answer - that's part of what makes brewing so fun! Feel free to call or email our brewmasters and they can help you plan out your next batch.