You sort of can, it's fairly common to have a product store already outside of Kentico. In these scenarios, you need to keep copied of your products in Kentico and create an integration task to keep Kentico's store of information up to date. This is because the Kentico API in the basket and checkout etc are all very dependant on the teh product data existing in Kentico. You certainly could do alot of modification here to make the checkout process work etc, but by that point - quite frankly - you amy as well write your own bespoke ecommerce module.
Juts because you have the products in Kentico as well, doe not mean you ned to manage them in here. You still end up storing some product content in Kentico, but your external system does all of the management and simply take updated form your integration work.