You can integrate external CKEditor plugins using http://extensions.xwiki.org/xwiki/bin/view/Extension/CKEditor+Integration#HUseadditionalCKEditorplugins but before you do this you need to think about how it will be saved as wiki syntax, because whatever you do in CKEditor is saved as wiki syntax in XWiki. So your first question should be instead: “how can I mention the user’s name in wiki syntax?”. XWiki syntax version 2.1 (latest) doesn’t support this out of the box and currently the only way to extend the syntax is through rendering macros. So you need to create a rendering macro that would be used like this (just an example):
{{mention user="mflorea" /}}
It doesn’t look sexy but it’s the easiest solution. If you do this then you will be able to insert a mention with the macro button from the CKEditor tool bar, or use a dedicated tool bar button http://extensions.xwiki.org/xwiki/bin/view/Extension/CKEditor+Integration#HPutDedicatedInsertMacroButtonsontheToolBar .
Hope this helps,
Marius