Like in Outlook, we can add multiple accounts, to access more than one through the same interface...?
No, not within the same session. You could have to set it up for login to multiple hosts and log in to and out of each one. It's theoretically possible to move between different accounts like this, but it's awkward. Similarly, this feature could be implemented, but would require some major modifications and not the sort of thing you could do with a simple plugin. Maybe you should post this to the Requests forum under Latest Release (http://roundcubeforum.net/forum/index.php?board=5.0) as well.