Currently there is no integration available to add a SharePoint on premise library or site as storage like we have a connected SharePoint online with Microsoft Teams.
I don’t think a feature like this will be released soon as well but there is an easy way to add a library or site as Website tab in any Teams.
This of course does not provide you seamless integration but atleast provides you access to on premise SharePoint site or library inside Teams.
You can view this small video to understand how to do it, you can add any website SharePoint site, library or any other site
Connect with me on Twitter or Linked In if you need more information.
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