Hi @Willem -
This is indeed already possible, and some users of ArduSub are also using external IMUs!
From the BlueBoat FAQ
Any NMEA position stream can be parsed by the Navigator Autopilot. To receive the messages from a network-connected device simply configure the Autopilot parameters GPS_Type to NMEA, Serial2_Protocol to GPS, and verify no other Serial port # protocol is set to GPS. Finally, under Autopilot Firmware, Serial port Configuration, set Serial 2 to: udpin:0.0.0.0:27000. Then direct your UDP NMEA stream to the IP address of the BlueBoat. Note that these NMEA messages are required for navigation: RMC, GGA, VTG, HDT. This approach uses the ArduPilot driver to interpret the NMEA messages - you can also use the NMEA Injector in BlueOS, which has support for fewer messages. If using a serial connection to bring the NMEA position data to the Navigator serial port 3 or 4, set the appropriate Serial# Protocol parameter to GPS and restart the autopilot. NMEA messages should be sent at 10hz or faster.
This link also has more information!