In my MK mugen the statedef that correspond to the guard system are from 120 to 155 in the common.def.
In my case, if I wanted to do what you say, I should go to statedef 130 and:
; STAND GUARD (guarding)
[Statedef 130]
type = S >>>>> change to C
physics = S >>>>> change to C
ctrl = 0
anim = 130
disable this:
[State 130]
type = ChangeState >>>>>>> null
trigger1 = command = "holddown"
trigger1 =! inguarddist
value = 120
THEN, IN STATEDEF 152 :
change the animation:
; CROUCH GUARD HIT (shaking)
[Statedef 152]
type = C
movetype = H
physics = C
velset = 0.0
anim = 151 >>>>>>>>>>>> 150 (stand guard anim)
ctrl = 0
and finally change the value to statedef 130
[State 152, 3]
type = ChangeState
trigger1 = time = 15
value = 131 >>>>>>>>>>> 130 (stand guard statedef)
I hope this helps you or gives you an idea.