Are siege unit abilities hardcoded?

Hello everyone. I was adding a new catapult grape shot to Mangonel / Trebuchet but i cant toggle the ability. The unit fires all its ammo and then switches to grape shot.
It looks like the availability of abilities is hardcoded, that means only Trebuchet can have additional stat_ter tertiary ammo (the cow) and Mangonel cant have anything, also the catapult fire ammo seems to be hardcoded. Is there any known fix or do i need to live with the tertiary ammo for Mangonel being impossible to switch?