Can I install a communication module on a DR series expandable model and perform communications?

Yes, as long as the style number of the communication module is less than or equal to that of the main unit. The style number is printed on the name plate of the main unit and communication module. The main unit's name plate is back behind the chart cassette (remove the cassette to view it).
To check the communication module, hold down the FUNC for 3 seconds, and then select YES under RE_SYSTEM to perform a system configuration.