Of course! Using a loop I can generate a number of slugs equal to a register. I can make that register equal to anything. Here's some example code which should do the trick (haven't tested). Every 2 minutes, it spawns one more slug than it spawned last time.
The disadvantage of such a script is that if you leave it endlessly you will end up with 999 or more slugs on the map, which isn't ideal I was considering the trigger for an invasion would be when you teleport down anything (raider/building/vehicle): each time you do so it sends slugs equal to the number of stuff you've got. I'd accomplish this using something along the lines of:
Untested and incomplete, but the idea is there.
N.B. If you are using a loop and the exit condition fails for whatever reason (eg there are no slug holes on the map thus GenerateSlug does nothing), keeping the scripter stuck in the loop, the game freezes up. Be careful!