This is not possible out-of-the-box.
I have consulted with our eCommerce dev team and they explained that this requires quite a lot of modifications as you need to extend all widgets that could have adding of items to a shopping cart. We also consider that this could be valuable feature to our product, so I have logged a feature request in our feedback portal for implementation in some of our future releases:
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items