Customizing search to add Sitecore results

We want a Zimbra search to query both Zimbra and Sitecore. We've got this working in Solr, but the missing piece of the puzzle is how to update the Social search results UI to show a new facet called "SageCentral" next to Content | Groups | People. Where do we go to customize this part of Community 8?

I also imagine that we'd update the (In) (Anywhere) buttons too so that there is a (SageCentral) facet.