New units can be created by cloning existing owned SKUs and modifying them. To create a new unit for the catalogue, follow these steps:
Navigate to the SKU you wish to clone and modify (ideally one that is as similar as possible to your desired SKU).
Clone the SKU.
Select the Edit SKU icon against the cloned SKU's thumbnail and select Edit SKU Structure.
You will enter the Seller portal, wherein you can make the necessary changes to the SKU.
Save the changes and open any design. For an already open design, refresh the page to load the change.
You can now find the new SKU in your design carousel.