I saw a link somepace where a guy did just that with a portege battery. Apparently the cells inside are standard types that you could pick up in a radio shack type place. As a matter of fact there was a post some time back by a guy sho did just that with a portege battery. AS a matter of fact, he even used a greater number of cells and dended up with a battery that lasted even longer than the standard batteries.
Do a search in google for reconditioning laptop batteries or something like that (I'd see if I can find it but I'm a bit busy today). I came a cross a website once upon a time that went through the steps in how to do it. But be careful, if ou get the voltage wrong you could damage the laptop.