Hi Britt,
you created the field as a 'customer extension field' to be shown on the UI and the user has to enter it, however it is also possible to add the field using the "SAP Cloud Application Studio" (PDI extension) and to perform a '30-day overdue calculation' during each save of an expense report and to fill the field based on the result of the calculation. Both kind of extension work the same with regard to the approval process setup.
The problem I see here, is that you need to be a partner for a CfTE-customer to be allowed to use the "SAP Cloud Application Studio", customers are not allowed to use this SDK and need to sign a contract as a partner.
Regards,
Peter.