Danny,
Yes, this is exactly what the "syntax hint" field in the input is for. Fill it in with something like matrix([?,0,0],[0,0,0],[0,0,0]) (well, you have a 6*6 version)
You might, as I have done, want to put in at least one ? to make this an "invalid" response, otherwise a student who navigates away might have their matrix marked as "wrong" (depending on your settings and penalty scheme). It is generally sensible to make a syntax hint "invalid" as strange as this might sound at first reading...
Chris