How to add reason code in Account Assignment Tab of Purchase Order

Hi All

I have a scenario, where user want to change the account assignment in Purchase order and when they change they need to give reason code in Account assignment tab.

If we see the standard SAP, we don't have additional field in the tab.  Please could you let me know how to add Reason code field, where I can add the reasons and when the user change the cost object or Account assignment cat. they need to select the reason code by drop down button in the same Account Assignment tab of item level

Thanks & Regards

Gopinath