Custom animation transitions

Is it possible to have a custom transition animation in mecanim? Example if you have idle, running and crouch states, pressing crouch while in running state will play an animation of the character falling and then transition to the actual crouch animation. Pressing crouch from idle state will just go to crouch state like normal. I know you can have a bool like "isRunning" in mecanim but as I understand it you can't use bools like this in blend trees, is there another way of achieving this in mecanim?