Board Thread:Support Requests - Designing Your Wikia/@comment-24658545-20141016184632/@comment-452-20141016211041

Yes. It is permitted to add content to the sidebar - but not to displace ads or remove default content. Excessively long additions which would move the default content all the way off the page are also not allowed.

See http://tardis.wikia.com/Special:Random for one such example, although there are other wikis with similar additions.