I'm no pro, but can't you force it ti constantly play the crouch idle animation when it moves?
Look at the Slippery Player-type
Have it change the friction once you start and stop crouching
I'm not sure exactly what you're going for here. Can you be a little more specific as to what you want the player to do? If you just want players to slide around on their bellies, change their crouch speeds to be higher than their normal speeds.