The 2-wire RS485 is inherently half duplex and there is no way to convert it to full duplex unless you are tapping into the IC pins of the RS485 driver chip (you could kill the CPU pin if a wrong voltage is connected to the pin so we are not recommending it)
It is probably simpler to just use the FMD88-10 to get your full duplex RS232 port.
Although if the communication between the PLC and your device is a command/response format, it should work with half duplex protocol. However, I don't know enough about your device to draw any conclusion but assuming that you have carried out enough testing to ascertain that a full duplex communication is required, then it is better to use the FMD88-10 or T100MD888+ PLC.