I've looked into what it would take to make a USB adapter for the C64, so I have a little background on the subject. This might get a little technical, and it looks like English isn't your first language, so if you need me to say something a different way, let me know.
Though it only uses four wires (+5VDC, GND, and two data lines), USB is much more complicated than RS-232. It's a whole networking system, only for devices instead of individual computers. Part of the USB standard is the distinction between "hosts" and "devices". There are also a set of "HID" standards, so all USB keyboards speak the same way, like all USB mice, and USB disk drives.
If you were to plug a USB to RS-232 adapter into the Amiga, the USB side would still want to be a client to some host, and it would only know how to speak the protocol for what kind of device it is.
Interestingly, USB On-The-Go (a new USB standard) is more of a peer-to-peer architecture, but there aren't a lot of devices out there using it.
