Well the best way would involve finding the right bit flag. But thats a deep and dark territory that few have entered into. You might check with DrPraetorious or lch. Or maybe ask on the Dom3Mods forum.
An easier way is to find one already in the game that you feel can be hijacked and make changes along the line of what you have done. Some searching might find something with less impact.
The ownership will tend to be Independents (nation 25) or Special Monsters (nations 23 or 24). I think SpecMons was added specifically for that purpose and there might be a reason to use it for all such attacks.
It would be great if you can do this. Such a spell would be a fantastic addition to the various GMing mods as another way to add special events into the game.
SideNote: this reminds me that the bit flags were something I considered playing with in my own special way. Randomly selecting. Maybe the mod for Chaos Nation could include some national spells with randomly selected bits. If they did something interesting I could record it for later exploitation
