Usually, you would create custom fields of the table "product" via the menu Display->Custom fields in order for you to fill these fields for each product and then they would appear automatically on the comparison table.
Now if you want to add the description, that will require to edit the file "compare" of the view "product" via the menu Display->Views in order to add custom code similar to the code you can see there for the product name in order to display the description.
