Well you can't really modify a cymbal. You can tune drums, obviously, to make them sound different, but cymbals are pretty much set in stone (or metal

).
Cymbals mature sonically (I have heard), with age. Some people swear by never cleaning a cymbal because the dirty cymbal has a better sound. I have never cleaned my cymbals and I dunno if I ever will, unless I get another set of them and can experiment.
The only other reason I can think of that would cause a cymbal to change sound is a crack or warp. You don't want to do purposefully hurt your cymbal, of course, but maybe you could try taping a tissue to it and just draping it over the side. This might make it less sustainful, but it might also make it sound worse.