Microsoft Outlook doesn't have a "unified inbox" built in, but with the use of VBA and a search macro, it's pretty easy to create.
A great article has been posted by Paul Braren to show the exact steps to follow in Outlook 2013. These same steps work perfectly in Outlook 2010 and Outlook 2016 as well.
The "short story" - create a very simple macro that searches all of your folders with the word "Inbox" in the folder name and present a single, unified look into it.
So, if you want to create an "Inbox All" unified inbox in Outlook, follow the directions and it works great.