I have never tried it. SWMBO loves the soda stream more than anything else we own, and if I managed to break it, I would be in the dog house for a long time.
I do know that you will void your warranty if you use it with anything other than water. So they don't design it for these purposes.
I also know that my brother once tried to carbonate water with some of the soda stream syrup in the water, and it made a huge mess. As soon as he pulled the bottle off of the machine, cola spewed everywhere. That is why you have to add the syrup after you carb the bottle.
So based on that, I wouldn't expect it to go smoothly. But as I said, I have never actually done it, so it might work.