You can't un-end an ended game. The best alternative, which I did once with an opponent after a mouseslip in an interesting position, is to followup with an unrated game, and just agree to replay the game to the current position.
Extending Timed Games?
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Is there a way to extend a timed game? In other words, once the game is over by time, and the ratings logged, can players agree to continue playing to see who would win if the game were not timed?