Hey Columbian - I'm still a total noob to cider brewing, but I've done a fair amount of reading on this and I'll pass on what I've learned so far. Just a warning though - this works if you want a sweet carbonated cider, but won't work if you want a dry carbonated cider. If that's what you want, you can read on if you like but it probably won't help you, lol.
- The preferred ingredient for back-sweetening/carbing cider seems to be apple juice concentrate. I haven't seen anyone say this but I'd assume you need to make sure it doesn't have any chemical preservatives just like the cider you started with. A couple sources I've read say to start with half a tube (so 6 oz.) of concentrate per gallon (a lot of people add the same amount at primary as well). I would add it to a new carboy and siphon onto it before bottling or something, as it would be a pain to divide it out and add a dose to each bottle.
- When you bottle you can use a cool simple trick to know when you have enough carb. You fill a plastic pop bottle (like a 16 oz Coke bottle or whatever) with your cider at the same time you bottle, and keep it with your bottles. When the plastic bottle is hard, like it would be before you first opened the Coke, you know you're good to go and it's time to cold crash etc.
- Just remember that cold-crashing won't actually kill the yeast but only stop fermentation while the temp stays low. If you want to store them outside the fridge you'll need to warm-water bath pasteurize or something, which I don't know a lot about yet.
Anyway hope this helps; if not I'm sure some of the more experienced brewers here can help out.
EDIT: Almost forgot - from what I've read adding more yeast should not be necessary!