It looks like this is coming:
“We’re in the process of finalizing designs for a Planner add-in for Outlook. From the add-in, users will be able to create tasks for Planner without having to leave their inbox. The email subject and body will autofill as the task title and description, respectively, and users will have the ability to set plan, bucket and assignees . We appreciate your feedback and patience and look forward to this first step in what we hope to be a greater integration with Outlook!”
Source:
https://planner.uservoice.com/forums/330525-microsoft-planner-feedback-forum/suggestions/14767056-create-task-by-sending-an-email-to-the-plan
