Yes, it is possible using 12c Oracle integration Adapter for SAP. We need to set the JCA property EncodedIDoc = flatfile, in the SAP Adapter design time wizard. At run time, Adapter will send IDoc in flatfile format.
Please refer SAP adapter User Guide for more details.
Oracle B2B only provide protocol to access file or send a file but for other related/ complex things are suppose to be done at Oracle SOA Suite. None of Oracle Apps adapters will be available at Oracle B2B including SAP Adapter. SAP adapter can be used with SOA, OSB and BPM .