It is easy if you create the following macro on all characters and map it to a key.

Then use the "keypress-send to all - tool" of your choice like HKN, ISBOXER, Keyclone ...

look up the round-robin keypress function in that tool :

isboxer: steps
hkn: Toggle
Keyclone: ??? (dunno)

and you are ready

Code:
/stopcasting
/cast Thunderstorm