Can we add a product/variant more than the available quantity to quote? If yes then How?

Yes, you can do this on your own in Quotilio. By default, if inventory tracking is disabled for a product then you don't need to do anything. You can add this type of product and its variant more than the available quantity to quote. But, if a product has inventory tracking enabled then either you have to disable the Inventory Tracking or have to increase the inventory of the selected product/variant.

Actually, we are using BigCommerce API to process the quote and BigCommerce doesn't allow to add more than the availability of the product/variant.

In Quotilio, if a product has inventory tracking enabled and out of stock then you will find the inventory edit option for the product.

Simply click the link and scroll down to the inventory option to edit the inventory.

That's it.