as much as i hate modded stock cans, if you want to do it, pull one end off, take the guts out, get a piece of pipe and drill an assload of holes in it (or, you can get some very heavy mesh and make a tube) put the tube in the muffler and pack a crapload of heavy guage steel wool around it, and put the cap back on. make sure the tube in the center goes inside the inlet and outlet a bit so that it won't move around inside the muffler.

thats pretty much how all aftermarket cans are built anyway.

again, it's not the same as a good aftermarket muffler, but if all you want is a lil more sound, it'll work. pack as much steel wool (or go get some real muffler packing) as you can, and it won't be TOO loud. i've done it for a few people, not my favorite choice, but it does the job.