Are there any sites (or people) that could show me how do this? I'm interested in buying an Epiphone Valve Jr., but I want to use my old combo as a speaker for it. Thanks in advance.
Yeah, I don't think it'd be hard at all. Open up your combo and see what the impedance of the speaker. The Valve Jr. seems to be pretty versatile impedance-wise. Then you'd just have to install a 1/4" jack somewhere on your combo and attach a couple wires.

EDIT: How good it sounds is going to be another story. What was the power rating on your combo?
MG Free At Last
Power rating as in watts? 38. And this will just be a kind of quick-fix until i can afford a decent cab.