<p>It is possible, but it has to be done by a moderator, unless it occurs naturally after 5 edits by 5 different people.</p>

<p>As jokerdino pointed out, you can get more information from the Stack Overflow meta <a href="http://meta.stackexchange.com/questions/11740/what-are-community-wiki-posts">question</a> on the topic.</p>