Prompt message to display while creating 'Existing Job Requisition' and don't have position code.

I have a requirement where I create Job Requisition using Existing Requisition Option and if the requisition selected doesn't have a Position code associated with it then it should not allow me further to create that requisition. This was they want to avoid creating requisition which doesn't have a position code.

But there are various requisition created without position code in our system. So when I select an exisiting requisition option and enter the requisition number which doesn't have a position code can I create a prompt message in that page saying this Requisition doesn't have a position code?