On the
Sales > Order > Create Order
I would like to add a serial number of a product.
Anyone any idea how to achieve this?
Define a custom option to your product (which is hopefully only sold on backend) and name it "serial number"
If it is not only sold in backend, create a config product, which has one simple item, the one of the frontend. Then you can add the custom option to the configurable product.
This doesn't look like in your screenshot but should do it for the process