As far as I'm aware you can't, but you can put umpteen actors on a map, and set them as deactivated. Then as and when you need them set the triggers to make them stationary anywhere.
Set up your player indexes so that you've got them in little groups (however many you want to activate at a time) i.e. 3 actors per player index, then put them on a neutral team and set the computer AI for all but the first 3 actors to deactivated.
Then when the first 3 die have a trigger set up to check to move the next player index (the next 3) to stationery at anywhere and change their team from the neutral one to the hostile one.
Set up the rest of the triggers the same, but adjust the player index each time.
You'll also want to deactivated each player index once each group of actors gets killed otherwise your map will quickly fill up with dead bodies.
The actors will not spawn in the same place unless you set up a waypoint and then as well as moving the player state to stationary move them to that waypoint as well. Though be careful you don't teleport an actor into a wall or other tile as this might cause the game to crash.
Otherwise if you set triggers to always I think they reset if you leave and re-enter a mission. Which might resurrect the dead, but I've never tried it.