Way to make Opportinuty Number (OPPID) available during Order Entry P4210/P42101?

During Sales Order Entry I would like to make use of the unused Opportunity Number (OPPID) field on the Order Header or at the detail lines.

I see the field in P42101/P4210 order entry but it is greyed out.  I cannot find a processing option to allow entry of data.

Is it possible to open up that field for Order entry associate to enter data and/or update that field.

Having this field available to the Order Entry associates would allow then to enter the SalesForce.Com opportunity reference or any other Sales/Marketing Program ID.

This would allow 360 view of customer from marketing campaign, lead, opportunity to actual revenue generated by relating the OPPID in JDE with the ID in SFDC.