Well, they have few keys, so they obviously can't play the full range at once. There may be a setting that allows them to move the keyboard up or down an octave to get more notes, but this isn't a given. Have you tried tossing off an email to M-Audio or reading their FAQs? Or hit a local music store. If those folks don't know, they'll know where to ask and find out for sure.
Sure you can buy a MIDI keyboard, but there has to be a way to get the MIDI signal into the computer. There are a number of MIDI>USB devices out there, if MIDI's all you're interested in adding. Or you could get something like the Tascam US-122, which has 1/4" and XLR inputs as well. HTH