Hi @Celini
Thanks for using our premium add-ons. Do you mean when you do and order from the backend? If you do, a blue button called 'ADD INVENTORY' should appear to add other inventories of a product to the order. You have to click on the pencil icon you will see on the top right corner of each product in the order.
And regarding the question about editing order numbers, actually, this is a Woocommerce feature so ATUM has nothing to do with that.
Best Regards,
Andrea.