a USB thumb drive needs 5.1VDC and the digital information needs to be read by a circuit. the HU has to read the drive to determine what songs are on it.
a 3.5mm port is fed an analog audio signal. it's a dumb input and cannot control the drive.
two completely different things.
what you need is a portable mp3 player that you can put your USB thumbdrive music on. that way the player acts as the reader/controller and gives your head unit the information into the 3.5mm port.