Now that Microsoft Teams is generally available, I wanted to provide a video demo showing how you can import an existing Plan into Teams.
Here are the steps and important items to not...
Updated Mar 28, 2017
Version 2.0
Blog Post
Hi Peter Atherton, thanks for the response. Unfortunately i don't get the option in teams to 'Add Microsoft Teams to an existing Office 365 group'. I do with my personal tenant, but with the work tenant, i don't. What's bizzare at work is that for some reason no users have the capability to create a 365 Group in Admin Center. However, we can inadvertantly create a group by creating a new team. The boys in our IT department apparently have global admin but are still unable to create a 365 group in admin center.
*edit: interestingly enough. I tried again adding a planner tab via MS Teams (choosing an existing planner). I did this before but tasks weren't appearing in Planner Hub (which was a bummer). However, this time everything seems synchronised and is appearing in hub! I've tested tasks in Teams and on the planner website, it's all in sync. I'm positive i did nothing different but at least now its working.