There are several ways folks are getting their USB CDroms, Hard Drives, and other drives to work in DOS. One is using what's known as DUSE driver system. The second method is using what looks somewhat like standard CDrom driver files and a USB driver. Both methods appear to require loading an usbaspiX.sys file first in config.sys in most cases. The third method is a bootdisk from a USB vendor. Method 4 is getting good reviews also.
Guide: DOS USB Drivers