in reply to
Re^2: USB to serial "coupler"?
in thread USB to serial "coupler"?
You didn't mention your OS, but if you are on Win32, com0com provides an easy way to pipe stuff through serial ports. It provides a pair of connected virtual serial ports.
You'd still need to write a translator that can talk to the USB dongle.
Your serial requiring legacy code would connect to one port. Your translator would connect to the other port. The translator would then talk to the USB dongle and perform any needed protocol conversions.