There are indeed 8-speed freewheels but they have a tendency to break axles.

How easy this change will be depends on the rear spacing of your hub and frame (126 mm, 130 mm or 135 mm) If it's 130 or 135 mm it's as simple as substituting the wider freewheel, respacing the hub, recentering the axle and redishing the rim. If it's 126 you need a new rear wheel and might as well buy a cassette hub while you are at it. Also you will have to force a 130 mm or 135 mm hub into the dropouts and, since the frame is aluminum, you can't cold set (i.e. permanently bend) it to the wider spacing.