Products - Add to cart
It depends on how your node 'products' are setup. In Drupal Commerce, 'products' are a fieldable Entity Type and each bundle of that entity type is a different product with different sets of fields. 

One possible solution would be to check out the Migrate and its ecosystem of other modules to see if there would be a way to convert your product Content Type to a Product Type.

Note that in the native Commerce workflow, you also need a Product Display node to act as a container for products to display them to the end user. Think of the Product entity as a single product option [in the example of a t-shirt it could be "X-Large Blue"] and the Product Display node is the wrapper that contains all options to display as a single "product" to the user.