Depending on the amount of headspace a little water would probably not hurt. Say, removing a couple inches space in the neck of a carboy. I'd boil the water first.
As far as using store bought cider ... I can't see why that wouldn't be fine. And yes, adding juice may start fermentation again (the store bought cider may too if it is anything but dry).
Another possibility ... especially if you intend to make cider or wine again, is to start on your collection of smaller jugs now. Very useful stuff.
If your carboy is 5 gallons, that is 640 ounces (128 x 5) ... assuming you will leave some amount in the bottom of the carboy to avoid picking up sediment, lets say you get 630 ounces out of it.
Those "gallon" jugs that store bought wine comes in are actually not gallons, they are 4 liter jugs, which means they are 135 ounces. (not 128 which is a US gallon) ... so, you could potentially use 4 of those 4 liter jugs for 540 total ounces and would have about 90 ounces left over out of your carboy.
A two liter wine bottle would be about 67 ounces ... subtracted from the 90 ounces .. you'd have about 23 ounces left over.
SO ... if you searched around and got 4 of those 4 liter jugs, plus one two liter wine bottle (common at bars and restaurants) AND got airlocks for all of them (note that is 4 of one size and 1 of another) ... you could rack ... and then have about 23 or so ounces left over to enjoy as the "gratuity" ... your reward ... er, um - quality control.