I'd do it this way:
First off, give the object that you're spawning some behavior for movement. I like bullet for this case.
Every(round(random(1,10)) seconds ->
System Create Object on layer 0 at
X: round(random(ViewportLeft(0) + Sprite.Width,ViewportRight(0) - Sprite.Width))
Sprite -> Set Angle to 90
I hope this helps. This will create sprites above the play area that then move down across the viewport.