AME -APINV - how to base the approval on a custom attribute?

Hi,

            I am a newbie on AME customization , so please bear with me if I am asking something fundamental.

           I have to implement 'Invoice Approval ' through AME.

            Our  requirement is to send the invoice for approval to the PO requested persons department.

           So , I created a Header level attribute to find out the 'Person who created the PO'.

            Now , how can I tell AME to build approval list using this information and Position Hierarchy ?

            I have implemented this using 'Approval Groups' , but that requires lot of initial data entry, I am trying to avoid that.

Thanks