Basilis is right there. StuffPlug 3.5 allows you to edit your nickname just like you would edit your personal message from the contact list.
StuffPlug can be downloaded from the official site. Install it, then click the new StuffPlug icon in the contact list toolbar and choose Options. Then, in the contact list section, check "Allow editing of Display Name in contact-list". You'll probably want to give all the other options a try, there are some really interesting possibilities with this add-on!
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