Yes, it can be used as an extension cab. All you have to do is unplug the speakers from the DeVille (the plug is on the bottom of the chasis, left of the power tubes) and then plug the speakers into you new amp. A couple things to think about, though:
1) the 4x10 DeVille is wired in 8-Ohms, and the 2x12 in 4 Ohms; you'll want to make sure the speaker cabinet's impedenence matches that of your head's. Of course, just because the cabinets are wired 8/4 ohms, doesn't mean you can't change that around. They were wired that way to match the DeVille's load output.
2) The speaker wire on the DeVille is not very long, so you will likely have to buy a 1/4"-plugged SPEAKER cable (NOT guitar cable, even though they look alike) and a female-to-female 1/4" plug coupler. Radio Shack sells the couplers. Almost any guitar store should carry the speaker cables. (Note: I use a Hot Plate with my DeVille, and had to go with the extension cable/coupler route. Really, NOT a big deal.)
Hope this helps.