Isn't a kH of 8 safe enough to add CO2? I don't think you have need of putting baking soda into your aquarium, but half a teaspoon of it in your 2 litre bottle of CO2 won't hurt.
BTW, how would I use Baking soda to raise my aquarium water kH? Do I just add directly into the aquarium, or do I first dissolve in a cup, then add the dissolved baking soda directly into my aquarium? Or some other way? Is arm and hammer baking soda safe to use?