I am unable to add out of stock items to the basket using your extension even though back orders are allowed on our Magento setup.
We would like to confirm that our Wholesale Fast Order for Magento 2 extension only supports adding products to card through fast order form if the products can be added on their product pages (which means Add to Cart button is shown). In other words, you are only able to search for in-stock products on the fast order form.
For more information about product, please refer Magento 2 Wholesale Fast Order page.
If you need any further assistance, please feel free to contact us.
Please visit our site for more useful Magento 2 Extensions: https://bsscommerce.com/magento-2-extensions.html