Is the importing air carrier required to submit the air waybill record as a master express consignment air waybill (standard message identifier FXI/FXC) when the cargo will be transferred to an ECCF or hub?

Is the importing air carrier required to submit the air waybill record as a master express consignment air waybill (standard message identifier FXI/FXC) when the cargo will be transferred to an ECCF or hub?

No.  The importing carrier may transmit the master air waybill record with the FRI/FRC standard message identifier.  The express consignment carrier/hub may then update the master air waybill record with a standard message identifier of FXC to change the record type to an express master air waybill.