*.zit is another compressed file format, created by Stuffit. It used to be the default compression app for Mac, but now that OSX has build-in zip compression it's slowly fading in popularity - though still used. You can get it here for Windows as well: http://www.stuffit.com/win/index.html
I don't know if ID will do it, but I know for sure that Axialis IconWorkshop can do it - but it costs
http://axialis.com/iconworkshop/
Once you have the sit file extracted, you'll find resource files inside. You may have to rename them to something different if they're not named *.icns. As I recall it it's something like *.rsnc, but I'm not sure.
Here's a guide: http://www.interfacelift.com/icons-mac/articles/winxp.php
