Is it possible to take someone else approval before taking my approval in PO

Summary:

Let's say I have designed a approval workflow where when a buyer submits a purchase order it goes to A for approval and then it goes to C for final approval. Let's say here A wants to take approval of B in some cases so he routes it to B and when B approves it then it comes back to A for approval. Post A's approval it goes to C. How this can be achieved?

I am aware about reassign and delegate. But in both case action is delegated to someone else. Ad-hoc route will mandatorily ask you to either approve/reject it before routing it to some.