This is still being worked upon. I hope to have more details in the next couple months.
Please know the team is actively working on this feature.
This feature request is working its way through the backlog queue. No new details are available yet.
-WarrenIwan Bijmholt commented
I agree with the others. This suggestion is not completed. Add an existing library as a tab is something different then the possibility to choose an existing site when creating a team
Thanks for following our suggestion to vote this up on SharePoint. It worked! SharePoint has confirmed that they have already started working on this and I’ll be getting an ETA from them after the holidays. -Suphatra