Sending SRM PO as XML via SAP BC

Dear SRM experts:

I am trying to send SRM PO as XML to vendor via SAP BC and I need to format the XML based on DTD given by the vendor. My question is where do I do that? If it is in SRM, please tell me which BAdI I have to use. If it is in SAP BC, how do I have to do it?

Also, I will greatly appreciate a sample ABAP program to format the SRM PO into XML.

Thank you very much,

Agus