If you cannot add a USB card, you are left with 1 other option.
Set up a PC with USB ports as a server and connect between the PCs with a CAT 5 cross-over cable.
The primary reason that you do not see converters to connect a USB device to a parallel or serial port is that the older ports are too slow. There is no way to transfer the data at even the minimum USB data rate.
Why is it that you cannot add USB ports to your PC?
If you have a PCI or a PCMCIA slot, it is possible to add a USB card.
The next limitation would be your version of Windows. Most versions of Windows 95, and Windows NT do NOT have USB device support.
Chas