sales order header text to purchase order header text

Hello Experts,

I dont find the option of copying sales order header text to purchase order header text.

We have the option of copying sales order item text to purchase order item text (via text copying rule) and works fine.

But when I try to create text copying rule for PO header text, I dont find source object as "Sales Order"

Is this functionality not available in SAP?

Or I have to do it via enhancement?

Regards,

Dipak.