Alphanumeric numbering for Global Blanket Purchase Agreements.

Hello,

We have automatic numbering on in our instance for standard purchase order.s however we need the blankets to be having a specific alpahnumeric number .

Since blanket purchase agreements share the numbering setup with standard PO's ,how to do we go about giving our own alphanumeric number to a blanket ?

Any help is greatly appreciated.

Thanks in advance.