You can do it with JavaScript. It's a common request, so there are probably a thousand variation of it around Wikia.
First, make sure JavaScript editing is enabled on your wiki. If it isn't, drop a note to Wikia via your wiki's Special:Contact/general page asking for it.
Then, in MediaWiki:Common.js, put something like
username = mw.config.get('wgUserName');
if (username !== null) {
$('.insertusername').text(username);
}
If JavaScript editing isn't enabled, you find out when you try to edit. If you're comfortable writing JavaScript, feel free to rewrite it into whatever does essentially the same thing.
Lastly, on the home page, put
Welcome, <span class="insertusername">fellow</span>, to the [[logo]].