Depends on the amount of poke! Are you sure the speaker in the combo is up to the job? Often they are not. No combo will sound like a 4*12 cab anyway.

If you are set on it:

Get a non-technologically challeged friend to do a bit of soldering for you, to remove the existing jack plug, join a bit of speaker cable in, and re-attach the jack plug on the far end.

The stuff that Maplins sells as "HiFi" speaker cable will probably do.

Speaker cable is speaker cable - so long as it is heavy-duty enough. Hi Fi buffs go on for ever about "oxygen free" etc but it's mostly bullshit. For stage and PA use you can ignore it.