I upgraded an old Suteki bike from a 5-speed to 7-speed freewheel by replacing the axel (which was bent) with a longer one and spreading the dropouts for the required spacing for the new 7-speed setup. You can check Sheldon Brown's site on spreading the rear dropouts and you can purchase new axels here.
http://www.biketoolsetc.com/index.cg...-Axle&tc=Axles