ADB -> USB is third-party only, somewhat rare, and they only support input devices like mice and keyboards (no storage). I don't think Apple ever made them. The only one I know of is the Wombat ADB-USB converter. (You can find plenty of converters that go the opposite way around, letting you plug ADB devices into USB.)

You can theoretically put anything you want on ADB, but you'd need drivers to do anything but mouse/keyboard, and Wikipedia lists the "actual" bitrate as 10 kbit/s.