adding information to signed forms

I know the intent is to lock forms when signed, but we have a form that after it has been signed we need to add a number to it.

Specifically I sign a form to say that I have authority to request a purchse order, sen it to another department where the purchase order is created and number is added to the form, then sent to another signatory to verify and sign.

I can't separate the forms to Part A and B (asked and received a resounding definitely not...)

Is there a way that I can divide the form into sections, make fields editable after signing, or any other suggestions?

Thank you.