At least at the moment, there is no possibility to change the location of where the Modbus DB will be stored. However, you could write a script to periodically copy the existing database file to the SD card. The database file is located in the /tmp directory, and the full path should be /tmp/modbus_db.
Please remember that writing to/from an SD card very often can drastically decrease its lifetime.
Let me know if any more information is needed!