mix
Set a mix to do it.
I assume you are trying to get additional travel on the same channel. In other words the tank is set up to move the gun elevation and then fire when the travel gets past a certain point?
If so, just use a PMIX to mix the channel into itself. The normal movement of the stick moves the gun, when you get to the highest elevation, flip the switch, the mix kicks in and you fire.
Hope that helps, if not just ignore me