You need to connect a USB-UART dongle to the USB port (search for usb rs232 dongle), it will have RS-232 connector that you can use for serial connection to "Another device".
dmesg -w in a terminal before you attach the USB dongle and it will display the port that will be created for that device, the output will look similar to:
[ 1755.423293] usb 1-1.2: new full-speed USB device number 5 using dwc_otg
[ 1755.556242] usb 1-1.2: New USB device found, idVendor=6547, idProduct=0232
[ 1755.563308] usb 1-1.2: New USB device strings: Mfr=1, Product=3, SerialNumber=0
[ 1755.570666] usb 1-1.2: Product: USB-UART Controller
[ 1755.575679] usb 1-1.2: Manufacturer: ArkMicroChips
[ 1756.907633] usbcore: registered new interface driver usbserial
[ 1756.945573] usbcore: registered new interface driver usbserial_generic
[ 1756.963615] usbserial: USB Serial support registered for generic
[ 1757.013663] usbcore: registered new interface driver ark3116
[ 1757.044072] usbserial: USB Serial support registered for ark3116
[ 1757.069578] ark3116 1-1.2:1.0: ark3116 converter detected
[ 1757.097247] usb 1-1.2: ark3116 using RS232 mode
[ 1757.124270] usb 1-1.2: ark3116 converter now attached to ttyUSB0
Then you can use /dev/ttyUSB0 as your new serial port.
The UART to USB dongle should have TTL voltage levels on UART side (either 3.3V or 5V, depending on the "Another device").