1) Did you mean that you are not able to access usb port through hyperterminal?
Well, there are softwares like MTTY, PUTTY etc. , through which you can send data to usb ports.
2) I’m not sure about power the modem using self powered usb hub, it will probably work.
But RS232 is a serial to ttl converter and you need a different thing; usb to ttl converter !!