There is not a way to clump your products inside the shopping cart. However, there is a work around.
Using a regular department and content pages along with the alternative link feature, a product clump can be created.
Add a New Department
A new department can be added under Content Departments Add Department
Add Content Pages
New content pages can be added under Content Pages Add Page
Assign the content page to the 'Product Department' created in the step above.
Set creative headlines and teaser copy for the products (this will be the only information the viewer will see from the content page).
Enter the URL of the product page that has been setup in the shopping cart.
Users will see the headline and teaser copy and when clicked, instead of going to the article page they will be redirected to the shopping cart product page.
Set up the Clump
Create a new clump that pulls content from the 'Product Department'. See this tutorial on setting up a clump for instructions.