ummm, I may be wrong, but isn't a COM port limited to 128Kb/s??? Whereas a USB 2.0 port can go up to 480Mb/s (491520 Kb/s)?
I know they make USB to COM adapters..... I don't ever recall seeing it the other way. (I mean, gender changers may work, but I think the speed differential is going to be the main difference.)