How to add a additional field(Not DFF) in the Billing Tab at the header level (Contract Management)

Summary:

When we tried to add the filed through application composer, it says that it is a non extensible field, so help us how to add the field or let us know whether there is any workaround or not.

We have a PO Number Field, whenever we give the PO Number in the header tab it flows the data to Contract Line level as well, same we are expecting from this field as well.

In application composer not able to edit the standard pages. created a custom field, while trying to add it to the billing page, the edit option there is greyed out. How to add the newly created custom field to the page.