I managed to add Yammer tab (Group or Topic) into Teams via Graph API. This is a another great example how you can add apps to teams easily.
It involved a bit of hacking (retrieve the application adding block via Graph Explorer) so it is not as robust as I would like it to be. I did not find a way (yet) to figure out the encrypted part in EntityID and ContentURL. I suppose they do contain info about the FeedID in Yammer but there must be something else as well (network url or reference to that also).
So this adding only works for those Yammer topics and groups that admin (etc) has chosen beforehand and used Graph Explorer to retrieve URLS.