from looking up the soda maker on google and looking at the instructions, it appears that you use a syrup to flavor the water. I would guess that if you could get coke syrup it would seem possible to work just fine. You would need some trial and error to get the proper mix of syrup to water.
More than likely the only way you are going to get syrup from coke, pepsi or any large manufacturer is thru a bag in box. Mosy are five gallon although it is possible to find 2.5 and 3 gallon. Chances are that the manufacturer will not sell to you directly, you would need to go thru sams, costco etc to obtain them. also you may be able to persuade you local independant c-store owner to purchase one thru his store and sell to you. Be prepared to spend upwards of if not more than $50 to buy a box of syrup.
If what you did yesterday still looks big, you haven't done much today.