You can certainly make bulk carbonated water in a keg, this is standard in soda making. You don't even need to boil the water, just use reverse osmosis or spring water.
You'll find that there are plenty of soda concentrates available that are much cheaper and much higher quality than sodastream syrup. Go do some reading in the soda making subforum, and you'll be surprised at how easy and cheap it can be.
Primary #1 - Midnight Ryeder (Midnight Wheat and Rye)
Primary #2 - Florida Weiss
Primary #3 - Kane-DOH APA (Honey Citra APA)
Secondary #1 - Downtown Flanders Brown (brewed August 2012)
Keg #1 - Raspberry Florida Weiss
Keg #2 - Cinnamon Raisin Cider
Keg #3 - NONE!
Bottled - NONE!