it CAN be done, but not easily, and will still require cash and quite a bit of work. Not exactly recommended, but here goes:
find the real estate on the back panel of the chassis (removed from the combo) to add a 1/4" or speakon jack. install it in parallel (+ to +, - to -) with the leads that go to the current speaker. put the chassis back in.
that's the easy part.
NOW, you need to figure out what the current speaker's inpedance is, although i'd bet it's 4 ohm. If it's an 8 ohm speaker, you need to swap it out with a 16 ohm speaker. one that works with the existing combo's internal dimensions and sealed or ported construction (if it's a 4 ohm speaker, swap it out for an 8 ohm). you can then hook your amp up to another MATCHING IMPEDANCE cab and not blow the amp. you'll have about half the power without the second cab, but with the second cab, the amp will sound louder than it does now.
that's a whole lot of effort for not much better of a rig than the amp by itself. upgrading would make a more sense, IMHO.
i think the consensus would be to just get different stuff, and leave the amp as-is.