Looks to me that BMW makes a USB retrofit kit! You only have to look to RealOEM.com to confirm this.
This is the kit for no Nav or Bluetooth...
http://realoem.com/bmw/showparts.do?...14&hg=65&fg=95
I think you only need this part... #65410427467
I am not sure if you need those other parts or not. Make sure your car doesn't have any of those options - that they list.
On this page they list the USB retrofit kit for Nav or Bluetooth...
http://realoem.com/bmw/showparts.do?...25&hg=65&fg=95