http://forum.blockland.us/index.php?topic=292351.0 The set identity feature does what you want. It does not make it someone else's name on the server as in your request, but that is because when there are only two people it is easy to figure out who is who. This simply gives everyone a fake name.
The only point of this script is to annoy others. Why would you need that?
For a bit of fun.