AP Approval Workflow Template - How to add start approver that's not on the LOV

Summary:

I would like to add a custom start approver it is not on the list of the value on the template. How can I add it?

Content (required):

I would like to add the start approver to be buyer of the PO, the DFF attribute that we custom created. But these attributes are not on the list of the value on the template. How can I add it. (Please see the graph. For example, I want the start approver to be PO buyer. The Top approver is also the PO Buyer. How do I accomplish this in the template.