A shopping cart item is basically some SKU data (COM_Sku), and most times the Page Type information of the object itself (say it's a laptop, it may have the fields "Memory", "HDD", "Processor", etc).
So if it's a item-specific attribute (like Memory, or Processor), then you want to add the custom field to that Page Type (ex "Laptop"), then use a Hierarchy Transformation to set different Transformation Methods with different page types (like Laptop) that shows those unique fields.
https://docs.kentico.com/display/K9/Using+hierarchical+transformations
If it's a field you want to add that will be with every product, you can go into the Modules and adjust the Sku class i believe in the E-Commerce module and add the field there.