For each as subevent after 'System| every 1.0 seconds' or you could use instead of 'System| create object', 'Kanobär|Spawn Fireball ...', but then all of your sprites will fire at the same time.
If you don't want that use 'System| Pick a random instance -> System| Create object...' as subevent after 'System| every 1.0 seconds', that will randomly choose an instance that fire a fireball.