Don't think soda is added. I would say it's natural fermentation. Just mentioning this sussmost has me longing for a taste!
If I remember well, it originates in some parts of Dwitzerland only. If Ubu could take a break from his beerdrinking, he could shed some light on this important matter.
Hey, last night I did not drink any red wine, ahm, beer - no alcoholic beverages at all - by the way, "Süssmost" is just the family version of what in the eastern/north-eastern parts of Switzerland (but who cares, Switzerland is so small) is called "Saft" (short for "Apfelsaft"), and "Saft" is not sweet, but alcoholic, and therefore much better, see?
ubu