The only way to create inventories in bulk for all your products at once is by using our ATUM Export PRO add-on.
You have a detailed guide to do so, here:
For new products, you can do the same thing (through Export PRO) or, alternatively, use the "Duplicate" link you have available on every product on the WooCommerce's products list. This would duplicate the whole product (including their inventories) and you can edit the new product afterward.
I hope this helps.