PO approval workflow customization

Gurus,

Here is the challenge. I have a scenario of 3 director positions with multiple holders for each position (Director1, Director2, and Director3) and 2 buyer positions (Buyer1 and Buyer2).

We are customizing workflow to select a director based on the charge account combination no matter who the buyer that is submitting the PO. We are using lookup table to select the director.

In the lookup, valueset with a query based on the position level it displays list of directors to select from for a particular account combination.

With our scenario, as long as the buyer is assigned to the director, it works.  For director3 position, I do not have a buyer3 to assign. Valueset is not able to look for the director without the subordinate (buyer)