In theory, yes you can add a speaker...you can add as many as you want! Speakers are passive components, simply two wires that act as inductors. If you hook the two speaker wires to where the old speaker is hooked in (likely soldered), it should work...wouldn't recommend hooking in multiple speakers to the same point (or at least more then is already attached)
THE CAVEAT. Bigger speakers don't necessarily mean more power or better sound. You actually need more power to...well...have more powerful speakers. A bigger speaker could give that affect but not super effectively...if your circuitry can't push enough power, the bigger speaker won't sound louder. Also you may stress the Game Boy's circuitry trying to drive a larger magnet.
Clearer sound has more to do with speaker and circuitry impedance matching. Scroll down in this link to "Why Impedance Matching Matters".
My suggestion? If you want better speakers, make some sort of portable external speaker with amplifier. It'll prevent you from stressing the internal circuitry, and you can make a cool custom system. You can get all super fancy even and make a neat outer housing :)