Don't use the bullet behaviour at all, Just set the sprites X position.
Loop a creation event how ever many times for the amount of asteroids you want. Create them outside the window size(so if its 640x480, set their position to 700 etc)
Then when they pass out of the Left Display edge return them to the other side.
SO, you have
CreateSprite:Asteroid at 700+random(200),random(480)
Run event 100 times
:Add 2 to rotation(center the hotspot/image point for central rotation, off center for more interesting rotation)
Asteroid.X -100 DisplayLeft:set position 700+random(200), random(480)
:set Asteroid width to:50+random(20)
:set Asteroid height to:50+random(20)
That should get it going, and you can do all sorts of things with it, add more variation in size, speed, positioning, rotation etc