its easy, lets say you have it like this:
EVENT // ACTION
on click / [,] // character/ start attack animation
what you need to do is create another event, similar to the regular shooting, or standing shooting, but this time, add an extra condition, like this:
EVENT // ACTION
on click / [,] // character/ start crouchAttack animation
character / is animation"crouch" playing
you add it by right clicking in the event section, and select add another condition, on the same( click[,]) event