I was thinking of a Criminal attribute. One that would give a certain percentage chance of police showing up on any mission or scenario you play with a character possessing that attribute. Also, maybe a chance of "backup" police coming if the first wave is defeated. Anyway, it's just an idea I came up with, but have no knowledge of how to bring to fruition.
Zorro
You can do that already.
Customize a police chief or other police character with policemen set under that attribute that let's your characters come with minions. Can't remember the name. Then set the chief up under the Nemesis attribute. The chief will come in randomly during missions, and he'll bring the others with him.
I believe you were also thinking of private army. Thank you for the help. Now, if I combined 2 private army attributes together, then maybe I could......