Absolutely! Here is the definition of seltzer water:
"Seltzer water is just plain water that has been carbonated. This water, which contains no sodium salts, gets its name from the German town of Selters, which was renowned for its natural springs. Seltzer water was first introduced as a cheap alternative to sparkling mineral water — and it still is an economical option today."
So soda stream just takes regular water and adds carbonation via C02. Nothing could be more simple (or economical)