Possible? Yes. You would need to make sure that the keg is entirely purged of O2 first, probably a good idea to sanitize the outside of the bottles before opening them and then by putting the bottle in the keg, fill it up.
Contamination is one risk but this should minimize as much of that and oxidization as much as possible.
How undercarbed are they? Drinkable or no? May even consider popping the lids and throwing a carbonation drop in there. Is this a high abv brew? the yeast could have given out if so. Did you taste one? Is there still residual sweetness? If so you may just need more 70 degree or so conditioning time.