Option 1 - Try adding a simple syrup of sorts. Might work easily enough, but you will let some of the carbonation escape when adding it.
Option 2 - Add sugar or prime tabs/drops. Bad idea, could make a foamy mess when you add the sugar
Option 3 - Leave it alone. That's what I would do.
Option 4 - When the keg kicks, pour the bottles into the keg under a blanket of CO2. Too much effort IMO.