The short answer is no, you cannot index Yammer with SharePoint; hence you also cannot take advantage of things like ranking or promoted results, etc. However, you can use Yammer's extensive REST API (which appears not to have changed much since 2014) to effectively view and search Yammer content from SharePoint.
First remember that you must authenticate with Yammer (and get an access token) before you can make any REST calls, and that Yammer does not pick up Windows credentials automatically like SharePoint does. This in and of itself may be a show-stopper, since you probably don't want your users to have to "log in" to Yammer from SharePoint just to search it.
See more about Oauth2 and Yammer at the below page:
The Yammer search endpoint (https://www.yammer.com/api/v1/search.json) returns JSON that looks like this:
From this it should be fairly trivial to build a "search vertical" page that displays a certain sub-set of the results, for example, messages, topics, users, etc. But these results will always necessarily be separate from SharePoint's "native" search results.
SharePoint 2013: Deep integration of Yammer Search with SharePoint search.
Here is the documentation page for the Yammer search endpoint: