I did that recently but cannot recall what was the trick. I have several email accounts and they all end up into the same folder. When I converted a POP account a few months ago into an Exchange Online account, Outlook created a new folder on the left under the main one. I succeeded to move that Exchange Online folder into the main folder. Now, I created another Exchange Online account. So, again, it appears under the main folder. I also need to move that one into the main folder so I will have all emails going into the same Inbox.

Anyone remembers how to do that?