convert from serial modbus to ethernet through plc?
Not a Modicon expert but perhaps this will point you to a solution.  

First, I believe the USB port is for programming only, not fieldbus comms.  That leaves the 485 and Ethernet ports.  

Look in the modicon development software for a couple of function blocks:  modbus rtu (master and slave) and for modbus/tcp.  If you can locate these, you should be able to craft a modbus rtu to modbus /tcp converter.  Hope this helps.