Urgent : transfer of custom fields of a sales order from ECC to CRM

Hi,

We have a sales order which will be created in CRM and replicated to ECC.

We have added some custom fields in table VBAK, which we are displaying in the sales order additional data tab B. When the user saves the sales order,we need to transfer the fields back to CRM.

We are aware of the the BADI CRM_DATAEXCHG_BADI.

Also i think we need to conisder the structure BAPIMTCS but not sure how exactly we can use it.

On the CRM side these Z fields exist in the structure BAD_BUS_TRANSN_MESSAGE-CUSTOMER_H of the method CRM_DATAEXCH_R3D_MBDOC_FILL.

Can someone guidde us on how exactly do we transfer the value from ECC VBAK-Z* to CRM  value BAD_BUS_TRANSN_MESSAGE-CUSTOMER_H.