Posted Invoice Update - Add Extended Column

Does anyone know how to add a custom field to the Posted Invoice Update screen and make it available for updating?

The field does not affect posting rules, it is more of an informational field for the business that would be good to have updatable after posting.

I looked in the Customization screen at the fields that can be updated and was not able to see anything that would explain how they are updateable.

Any help is appreciated!