I have a 4 gallon batch of dry cider that has been eaten up by champagne yeast. If I just leave it in the carboy and then wine thief a bunch out when guests come over, add some sugar, and then throw into a seltzer maker, wouldn’t this be OK to serve?
If you’re talking about using a SodaStream… bad idea. I have tried this and if you carbonate anything other than pure water, it will spray all over your entire kitchen. Not a fun cleanup job.