We can add a third party MODbus driver to our FPA5000/1200 panel. The Modbus driver can provide either RS232 or TCP/IP to communicate with third party system(mainly for the third party BMS).
I am not sure it work in your case, it is because the connection is directly connect to other panel. You need to double check the third party panel is able to read Modbus protocol directly. For our panel for example, even though our panel have the Modbus module, but it is not possible to read message from the other incoming Modbus message.