I won't say you "can't" do it, but I can say that it would be highly inadvisable.
Using my new 15" PB as a reference, in order to get to the keyboard, one has to almost completely disassemble the machine to even get at the keyboard. I imagine there are backlit keyboard modules available on the 'net, but you would seriously have a difficult job of it, I should think.
If you want to know the whole procedure to install or replace things,
www.pbfixit.com has illustrated, free guides on how to install or replace just about any part.
Again, I really do not believe that this kind of surgery would be a very good idea. You would almost certainly void any warranty you might have, and unless there's already a handy plug available to power the keyboard light, you may need to do some *really tiny* soldering.
An alternative would be to purchase an external USB keyboard that has a backlight. I have one by a company called...Illuminex, I think. It has a soothing, adjustable blue glow. The keys are all translucent, though, so you don't get an effect as nifty as the Powerbook's backlit keyboard, but it's much less dangerous.
Just my .02!