com1 is for serial devices, usb headers are specially designed for front usb ports. If you want to connect a serial device to USB, there are Serial to USB dongles that you can buy.
You cannot have a usb port as a serial one. If your PC doesn't has any serial port available buy a USB-Serial converter. Though, be aware that no all the converters work with all the serial devices. I tried one with an old serial modem some years ago and I couldn't make it work...but a serial mouse did.
