Conceptually, it should work. Practically, probably not a good fit.
I'd be surprised if a RAID controller would work with USB drives. Software RAID seems more likely to work. But for RAID to make sense, you really need two, or three, or more drives online all the time. With external USB drives, you (or someone else) can come along and unplug one at any time. That would probably foul up your RAID arrangement; at a minimum, you'd probably lose your data redundancy. The portable, flexible nature of external hard disks just seems to me to run counter to the whole notion of a RAID array.