Hi Andre,
You need to identify the structure related to the BOL object BuilSEPAMandate.
Then add your custom fields using append structure.
you need to figure out the standard GENIL class linked to the BOL object.
Then you can inherit the GENIL class and populate the new fields in the MODIFY_OBJECTS and GET_OBJECTS method of your new GENIL class.
Regards,
Subhashini