In your weapon FSM, add an ALTATTACK state, and you can set one of the frame actions to ZOOM
Something like:
state ALTATTACK IDLE 0
frame 1 0.01 0 0 0 NONE
frame 1 0.01 0 0 0 ZOOM 40
frame 1 0.01 0 0 0 READY
And to have it reset when you're not holding right-click, do something like this in your idle state:
state IDLE NONE 0
frame 1 0.01 0 0 0 NONE
frame 1 0.01 0 0 0 ZOOM 0
frame 1 0.01 0 0 0 READY