I’ve seen that there already exist a few threads about this topic but as they’re pretty old I would like to discuss it again.
Is it possible or at least planned?
I saw some features in the email options for adding further accounts and identities and I wonder what they are for/if they are what I am looking for. My guess is no but who knows… 
I am in the process of getting rid of my own hosted services and switching to murena.io plus looking for an email service. I thought of using addy.io maybe along with murena.io but this makes it more complicated and expensive. Mailbox.org though supports custom domains but quite limited aliases and Tuta.com does not support importing/migrating mails yet. Yeah… 1st world problems I know…
Best would be having all under one hood at murena.io…