Can the Arduino Due be used as a serial to USB converter?
Yes it will work as a USB to serial device, however the serial will be at 3.3V not 5V. It'll still be serial though.

For it to send data to a 5V device, depending on the device, it might work.  3.3V is often around the "high" threshold for an input pin on a 5V device, but check the datasheet for the 5V device to be sure.

For receiving data the 5V will be above the 3.3V supply voltage of the main SAM3X chip, so should be avoided. As with most Atmel datasheets, actually finding the information you want is almost impossible, and I haven't found if there is a maximum "over voltage input current" or not, so to be on the safe side a full level shifting voltage divider should be used on the RX pin.