Just to be safe, adding a little new yeast probably wouldn't hurt anything. The real trick in my opinion, is how much sugar do you add above the required amount to carbonate? Until you have done it a few times, you may not be able to hit the nail on the head. Unless of course, if you are adding a non-fermentable sweetener, then just prime and wait, just like beer. But, a couple of things to think about: are you going to serve your cider at room temperature, or chilled? Here is why I ask, a liquid at room temperature holds less CO2 than the same volume chilled, but you already know that. A PET tester bottle can be a little misleading to the amount of carbonation just by squeezing it at room temperature. But, if you have a still unopened bottle of soda next to your tester bottle, you will be able to tell the difference between a "firm" bottle, and a rock hard bottle like it comes from the store. Ciders are funny things, sometimes they are fully carbonated in three days or less, and sometimes they take a week or more, hence the PET tester bottle. What I did was sweeten to where I wanted it, and then add the measured amount of priming sugar, and wait a few days.
My dishwasher pasteurized my hard cider, I know this is true, as my 1-1/2 year old batch, none of them have blown up.
