Hi Asha,
I think is very important to know HOW TO works SAP when your have that scenario. This scenario required implement objects like BADI, table, Enhancement, etc.
If you know the technical process for implemetn this process, will be more easy understand it, please check this link: http://www.abapgurus.com/how-to-add-custom-field-in-additional-b-tab-for-sap-sales-order/
I hope be useful for you.
Regards,