When you issue an order, the game uses a string called an order ID. This string is displayed in the ability's field values in the Object Editor under Order String, such as "slow" and "unslow".
Custom abilities still use the original ability's order string. It's only the GUI Trigger Editor that makes it seem as if there isn't such an option. Just order the unit to cast whichever ability the spell was originally before you edited it.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.