Ethernet and RS232 are very different in nature so there is no simple conversion. But it is very easy to convert the RS485 of the Nano-10 into RS232 port using the Auto485 (
http://www.tri-plc.com/auto485.htm)
However, unless size is a problem, it probably make more sense to get the FMD88-10 which has both RS232 and RS485 ports built-in.
But if your purpose is to use the Nano-10 as a Modbus/TCP to Modbus RTU Gateway, then the new Nano-10 can be configured to act as a Modbus Gateway, where any Modbus/TCP with ID that is not the PLC's ID will be sent out and received using Modbus RTU protocol via the RS485 port to other Modbus slaves. That is a new feature on our Nano-10, FMD or F-series PLC with r77 or later firmware. Contact support@tri-plc.com if need the specific instruction to enable the Modbus gateway mode.