As you correctly mention, USB is not COM. USB does not use COM port numbers. Therefore there is no way to do what you want.
1) Install the printer driver and use .NET's
PrintDocument[
^] class.
2) Or, find some product-specific non-standard way to talk to the printer directly using
#usblib[
^]. You will have to handle usb end points and connections and stuff yourself. I highly recommend the first alternative.