Non-indexed (friction) shifters ARE compatible with indexed derailleurs, you just aren't going to have indexing. If you wanted indexed shifting
(clicks into place), you would need new shifters, derailleurs, freewheel, and chain. You'd also probably have to mess with the rear axle a little. But I don't think you'd have any problem with the shifters on a triple in general.
To just put a triple on the front, you would need new front & rear derailleurs, the new crankset, and a new chain. There's a possibility you would also need a new bottom bracket so the new crank sits in the right place, but maybe not.
Ultimately, it depends on how radically you need to change the gears. If you just need to go a little lower, you might be able to find a 39 tooth chainring for the front. Then you wouldn't have to change anything else and you would get 7% decrease in lowest gear. If you can't find a chainring, you could get a double crank with chainrings that are better suited to your riding.
The easiest change is to swap the freewheel. You could probably go to at least a 14-28 in back. (http://www.bikeman.com/FW5005.html)
That would give you a 17% lower gear. You may be able to use the same rear derailleur, but check carefully how it handles the big-big combination before you ride it. Might also need a new chain but that's something you may want to replace regardless.
Note that you probably can't put a 6 spd or 7 spd freewheel on that bike without respacing your axle, redishing the wheel, and spreading the frame. None of these are major issues, but they would need to be done to get beyond 5 speed.
If you think a fairly moderate lowering of the lowest gear would do it, I'd say try the free wheel first. Remember that a 1 tooth change in the back makes a much bigger difference than 1 in the front. Look for a smaller chainring for the front if you need a bit more. Then look for a compact double or go to a triple if you really need a big jump.