This can be done! XFire has a new plugin which can connect to MSN servers. This means you can use the XFire client to MSN! And since XFire has support for world of warcraft you can use MSN ingame.
Get XFire
here(the plugin is automaticly included in the latest version) and make an account. Then go to options > manage accounts ALPHA(not sure if it is manage accounts, just manage something ALPHA) and set up for your MSN account. From that point on when you start XFire it will also log into MSN. Thats it!