OKS Invoice Print - Can we print the sales order number from which the contract was created

Hi,

We always create a extended warrantry from OM.

The invoice generated from Service Contract Billing has reference of the contract number. Nowhere in the invoice there is a reference of the sales order.

I am looking a simple way to print the sales order number on invoice print (without modifying the print program). I was hoping to change the Invoice Transaction Flexfield for context OKS CONTRACTS. I do not see one for OKS CONTRACT.

There is one for Line Transaction Flexfield.

Is there a way to accomplist this.

Thx

Nagarajan