If you can give a legitimate reason why you would want to do that, then maybe we can help you out. Otherwise, trying to obtain passwords of other users without their consent or without being the system administrator is considering 'cracking' and is against the rules of this site. Feel free to read them if you're unsure of why this kind of post would be considered inappropriate.
http://www.linuxquestions.org/rules.php
Thanks,
-CC