You can use the 'System: Set timescale' action to change the speed. 1 = normal, 2 = double speed, 0.5 = half etc. That changes the speed of everything in the game including timers.
You can also use 'System: Set object timescale' to change the timescale of individual players. Give each player a timer like the countdown timer. On key pressed - set player timescale to 2 and start the timer. When it reaches 0 - set timescale back to 1. The bar would be a Sprite or TiledBackground and you adjust it's width based on the timer value.