At the present time you are unable to add any additional columns.
Our products are developing constantly and to assist us in acheiving our customers requirements we encourage you to leave feedback from within the product.
Thanks for your reply. I'll send a feedback to the technical team. What is the email of the technical team?
I'm afraid the improvement will be added to newer versions, not my version Premier Edition 2018.
You can add a custom column, but it won't populate automatically.
There are a few included that you may not be using, including two Other columns, which you can rename on your invoice template for printing.
Or, add item custom fields, which can then be added to the invoice as columns on the invoice table.