FPGA NIOS II and RS-485
Adding a UART rs232 serial port interface is done through a software programming, allowing the device to transceive rs232 data communication. Thats the job of your device to send and receive data by rs232 communication protocol. 

rs232 to rs485 conversion is done through hardware implementation using one of the available converter ICs such as this one MAX3160 from digikey. 

So in software programming you still control your device to behave like its using rs232 protocol, but implementing rs485 conversion can allow your device to communicate with rs485 devices and extend your network from one slave IC to a certain number of slave ICs "according to the specification of the IC".