So YAT sees the Southco USB device as a regular COM port? Then it is easy.
You must find the voltage used on the Southco serial. It is probably full RS232 voltage which uses both positive and negative signals. You will need a RS232 adapter or shield to generate that voltage. It may be 3.3V or 5V, which is called TTL. The Arduino can do that directly, maybe with 2 resistors if you need to drop a 5V Arduino output to 3.3V.
Then you need to know the baud rate.
If you have an Arduino with multiple hardware serial, then use one of the TX pins. If not, use the SoftwareSerial library and choose a pin.