The short answer is: You can't
Long answer: Products and stocks are two different data domains and are separated to be able to retrieve stock levels independent of product information, e.g. by a web service. The second reason is that stock levels are highly volatile and they are separated from other product data to enable efficient caching.
But Aimeos is made for being customizable to your needs. You could implement a "stock" panel in the admin interface that lists the stock levels together with the basic product data and a link to the product in the product panel. That's rather easy to implement.