Can a approver who is not in the approval routing but has DFA authority on a higher level then that

We have a requirement that DFA holders on higher levels are able to approve purchase orders but also vouchers which are not routed to them but are routed to a lower level still have the ability to access the approval page and approve it.

Example is as follows:

PO is created for 40,000

is routed to approvers on step 4 of the approval process

A senior manager who has DFA for steps higher then step 4 wants to be able to approve the PO without being included in the routing.

Currently when the manager access the PO Approval page it is in view mode only and the approval buttons are hidden from the page.  The only action that the senior manager can do on that page is add himself as an adhoc approver which will then rout the PO to him after the approver