Chances are the combo wasn't designed for even, studio-monitor style response, so it won't sound great. Still, as long as the combo has an input that will handle line level from the mixer, it's do-able.
If your combo has an effects loop, plug the (line level) output from the mixer into the (line level) effects return jack. This should bypass the amp's pre-amp. Depending on the design, it might also bypass the amp's eq. What you're left with is a mixer feeding a little power amp and what's likely to be a fairly colored cabinet.