risky business.
Speakers design s a carefully orchestrated marriage of three things based in math and physics which takes into account the physical/electrical properties of the woofers (and there's a LOt more involved than simply fitting in the box) , the physical dimension of the box and the crossover.
If you get lucky, it may be almost as good as the original woofers but, unless there's something wrong with them, I wouldn't bother. IMNSHO, if you want more bass, gets subwoofer.
Here's a site on his to explore to get you started.
http://www.epanorama.net/links/audiospeakers.html