The bar is at 63 inches. Robot can only extend 57 inches. How do you close the gap?
The 12in extension limit is only in extension from the frame perimeter (think side to side and front to back). Once endgame has begun robots can extend as high as they want as long as they are in the rendezvous zone. This also means that the max robot height is 45in, both before the match starts and during the match. Per G17 and G18.
Cooper3175 has the answer, that you can extend upwards as far as you need to during the endgame, as long as you are in the rendezvous zone. But you also need to remember that the 12 inch frame perimeter restriction is not waived during endgame. While vertical extension above the robot is unlimited, horizontal extension relative to the robot beyond 12 inches is still illegal. So using a winch climber that turns the robot sideways as it climbs, for instance, could be very problematic, since you would likely extend outside the frame perimeter by more than 12 inches if you did that. See the diagrams in the game manual accompanying G18 if you need clarification on this.