Additional features, settings, or issues not covered by specific Microsoft Teams categories
"It's not feasible to merge the existing plans with existing Office 365 group or team. Because these plans are also Office 365 groups. When you created these plans, it automatically created the corresponding Office 365 groups. These plans are the instances associated with Office 365 groups. So you cannot add these plans as tab via Planner app in Teams. If you want to add a plan to Teams, it requires to be determined when you create the plan. For the existing plans, you can only create a new team from an existing Office 365 group. The corresponding Office 365 groups will appear in the list."
I still think it could be done by changing how the Teams Planner app looks at the group and somehow be able to associate the Plan even if its not part of the group.