Apologies for the previous irrelevant admin response. We’re still thinking about this one.Jonathan Isaacs commented
If you don't want to enable it through the UI, please give us a PowerShell cmd we can run!
Without this functionality, we cannot move distribution groups to O365 groups.
Thanks for your feedback. We are currently thinking about this. Can you please let us know what scenarios your organization uses shared mailboxes for and what options do you use. This will help us.
We have announced that Group Owners will be able to leverage existing groups to drive membership in Office 365 Groups.