Is there any way we could implement an workflow approval for the total invoice value (including tax)

Summary:

Hello,

We would like to understand if we can customise the BPM task called "InvoiceApprovalHumanTask" as to take into consideration not just the net invoice amount, but also taking into consideration the tax amount, thus the entire invoice amount shall be captured into the approval workflow and not just the net one.

We have also explored the option of defining an additional calculated descriptive flexfield at the invoice level which would contain the sum amount between the net invoice amount plus taxes amount, but based on our knowledge so far, we would think this is not possible as the invoices are automatically generated by the process called "Generate Invoice", thus we cannot intervene and add a DFF to a standard process.