how to transform a field from SRM local purchase order to ECC

Hi Experts,

I am new to SRM . Could you please help me on below issue

The value in the field on SRM local purchase order item should be transferred to a corresponding custom field on the replicated ECC purchase order.  Functional team asked me to use SRM BADI BBP_ECS_PO_OUT_BADI . How can i develop logic in this or do i need to create any RFC.

Thanks in advance.

kumar