My Receipts - without a purchase order number

Summary

Can I limit the values returned on the requster / preparer lookup ?

Content

Using My Receipts the user needs to know one or more of the purchase order number, requester, or entered by.

There are lookups available on all three fields, however the requester and entered by list every employee and the purchase order number shows all purchase order numbers.

Is it possible to amend either the lookups and / or the role that we are using to limit what the user sees or gives additional search features.

What  I am thinking of - could the employee lookup limit by something like the cost cost centre on the default expense account, and could a supplier filter be added to the po lookup?