Yes. You can add main categories and subcategories by mapping the category columns in your Data CSV to the Shotfarm Core Fields: Main Category, Subcategory (01), Subcategory (02), etc. during the Core Field mapping step.
If you are creating custom fields then you must have at least one main category as custom fields map to the lowest level of your product categorization (aka product hierarchy). If you don't map and upload to the Main Product Category field then we default to "General" as your main category.