spawn down

Why is not own.worldPosition.-z to make added objects spawn down possible?

own.worldPosition.-z

because that is not a valid code

you grab the .z like this
own.worldPosition.z

then you give it a value
own.worldPosition.z += 1.0

to go down you need a negative value
own.worldPosition.z += -1.0

or
own.worldPosition.z -= 1.0

Thankyou that answers my question.