Hi Manikantan,
I had implemented it long time back so couldn't recall exact steps. But you can achieve this by defining your custom field in "PunchOutSetupExtrinsics" group. Also, you may use a parameter PunchOutSetupExtrinsicsClass defined in Parameters.table file where you can define a java class. In Java class you may even filter out this parameter for some specific vendor. Java class should implements PunchOutSetupExtrinsics.
At the sametime, you will have to work with vendor to identify the punchoutkey.
Here are some more details about setting up extrinsics on Punchout request.
public interface PunchOutSetupExtrinsics: Interface used to generate customized extrinsics for the PunchOutSetupRequest. The server will call the implementation of this class based on the parameter Application.Procure.PunchOutSetupExtrinsicsClass in Parameters.table. The extrinsics generated by the generateExtrinsics method are sent in the PunchOutSetupRequest in addition to those defined by the group PunchOutSetupExtrinsics.
Parameter Application.Procure.PunchOutSetupExtrinsicsClass: Names a Java class that can be used to customize the PunchOutSetupRequest cXML document used when sending PunchOut requests to Ariba SN. In general, you add extrinsics to a PunchOut request by including them in the PunchOutSetupExtrinsic group. You use a custom Java class only for situations where you require more complex logic.
Thanks,
Amit