Try Leetificater Plugin...
http://www.msgplus.net/pluginsdb/down/?action=file&id=41
Or this other plug-in I've never tried...
http://www.msgplus.net/pluginsdb/down/?action=file&id=91
The only other decent one... it's not going to make your messages l33t... but it will ad some life to them... Reverse Plug-in
http://www.msgplus.net/pluginsdb/down/?action=file&id=85