Easiest (and most expensive) solution is purchase a counter pressure filler which is attached to your gas supply and beer delivery pipework, although this is probably over the top for just an occasional bottle.
How about chilling the keg down as much as you can, and upping the keg co2 pressure for a few days. Then reduce the pressure to a low serving pressure, and introduce into the chilled bottles with a tube with a tap? This should ensure it is sufficiently carbed when it returns to normal serving temperature.
Another very effective compromise (the compromise is the aesthetics), is the use of the following:
This requires you to use a plastic bottle, but is fully carbed up (without yeast deposit) as long as you chill the beer down as much as possible before carrying out the bottle filling, as you lose some pressure when you remove this fitting and fit the normal cap.
All the above is assuming that when you say 'keg' you are talking about a metal pressure keg (eg Cornelius keg), and not something like a King Keg, which might prove more difficult from the connections point of view.
