See the results of the below experiment HERE
I just dropped a coopers carb drop in to a well chilled 12oz undercarbed bottle of cranberry wit. As soon as I dropped it in, all the nucleation sites on the sugary lump caused foaming...but I was at the ready with the trusty red barron capper and new cap.
The foam reached the top of the bottle by the time I affixed the cap, and certainly would have foamed over if it was properly carbonated. This is probably best done with two pairs of hands...one to drop the sugar in and hold the bottle, the other to move in swiftly with the cap and capper.
Anyhow, I'll leave it out at room temp , giving it a shake every once in a while to rouse the sediment to see if the yeast reactivated and processed the added sugars. I'll pop it after a week to compare CO2 to another untreated bottle.
this is reallly just an experiment to show proof of concept. I hope I can do some nose thumbing against forum nay sayers who advise to just chalk undercarbed bottles to learning. Given my prior experience with culturing yeast from bottle conditioned beers, I believe this will work just fine. If I get increased CO2, then I'll use a few more bottles to dial in the carbonation by using munton's carb tabs, by adding 1, 2 and 3 tabs to each bottle. I would have gotten these to start the experiment...but the LHBS didn't have any when I went. bummer.