Oh yeah, I have that one.
Their instructions are:
1 gallon: 1/8-1/4 Tsp yeast, 1 Tbsp or 1/2 oz extract, 2 cups sugar, 1 gallon water.
2 gallons: 1/4-1/2 Tsp yeast, 1 oz (or half bottle) extract, 4 cups sugar, 2 gallons water.
4 gallons: 1/2 - 1 Tsp yeast, 1 bottle extract, 8 cups sugar, 4 gallons water
You can forgo the yeast and just use co2. I used sucrose (and a bit of maltodextrin for body) in mine.
I'm not sure how you would want to go about doing it though. You could potentially add the sugar right to the carbonated fluid and see if it will dissolve, but it could take a while. Perhaps superfine sugar (or just sugar that's pulsed in a food processor a few times) since that's supposed to dissolve more easily in cold liquids.
You could make a sugar syrup and add that, but it would water down what's in there. You'll lose a little bit of carbonation while it's open, but you can recarb.
If you were lacking the sugar, that should help, but if you're lacking more than just sweetness and sugar flavor and are actually lacking flavor, that will be harder. It would mean that you would want to use less water to the same amount of extract the next time.
A good test would be to use 1.6 oz (1&2/3oz) sugar in a 12 ounce glass and pour the carbonated liquid into that and mix and taste.
(1.6 oz was taken due to: recipe: 1 gallon batch uses 2 cups sugar, where the cups are 8 oz by both measure and equivalent weight. 1 gallon =128 ounces. 1/10th gallon =12 oz liquid. 1/10th of 16 oz= 1.6 oz)
If you already added 8 cups of sugar to the liquid originally, you should ignore what I've said since you would then be looking at varying the amount of extract and water.
Personally, I just made myself a simple syrup in a Torani bottle (with dispenser) that I pour into a glass and add carbonated water so I can vary the amount of syrup until I find how much is right to me.