Yeah, you cannot do this, but you are taking the wrong approach. You should simply create a playlist and drop all your klezmer-prog (or whatever genre you want) into it.

There's a billion ways to make playlists. You might use this for inspiration:
http://lifehacker.com/355743/top-10-...mart-playlists
In there it mentions using keywords to give the songs attributes, but at that point, rather than typing all that, just drag and drop the songs you want into a playlist and listen to it on shuffle!

Personally, I have playlists for Progrock, instrumentals, Driving, Cover versions, Recently purchased albums, Songs that haven't been played in 2009, Lounge music, Party songs, Songs my wife likes, and a dozen others. I use them all.