Unfortunately, that is not possible without software changes. RUTX10 Modbus slave functionality will only work with RUTX10 information only and it can't work transparently sending other connected devices information.
The thing you can do though is set-up RUTX10 as a slave device and one of the connected devices to it as a slave device too. (You will be able to use only one device connected to RUTX10 as a slave, as RUTX10 can only be one slave and can't be configured as multiple.)
Then what you going to do is port forward 502 port to PLC LAN IP port 502, so when Master is requesting data from RUTX10 - it will send a request to PLC instead.
Other way around is not possible without changes that will probably cost you - you should consider contacting sales manager about this if you're interested.