This is nothing to do with the "condition of presence" field in the unit detail screen, whichcontrols the likelihood of the unit appearing at the start of the mission.
What you need are createUnit, createVehicle or camCreate commands, principally createUnit. Check the online comref for an example. The best way to create a really large amount of lag really quickly is to create a lot of units at the same time, so be careful.
There may be some scripts in the Ed Depot, not sure. Have a hunt around.
I also wrote a tutorial on reducing lag, which you might find helpful.