You can, but not without a major -non feasible IMO- surgery.
Quite a few amp manufacturers have adopted the "no-extension-cab" feature into their combo amps lately.
Trying to go around that always means changing (or defeating) the internal speaker unless a series connection is made.
From their perspective it's a genious move, from the customers perspective not as much.
Most combo amps in the lower price range suffer from quite a few inadequacies, enough IMO not to be useful in anything else than as-is (practice) use.
That's a shame really, because even though I'm not particularily fond of combo amps, the Deltalite II modded SWR WM10 I sometimes use is an absolute killer for its size.
I don't have a compact extension cab for it, but with that it would take care of most of my small venue gigging needs with a 4 string.
If You like the tone that the Orange gives you, I'd mate a plate amp with a cab of your choosing and use that as an active extension cab if and when you need more volume.