Unify Calendars on 2013 for all imap accounts

I have setup outlook 2013 for a friend and also an outlook.com (hotmail.com) account , as i needed it for office install and also to sync all his devices.

the hotmail.com , I want to be the default calendar , so if he gets a meeting request from any account , gmail, hotmail , then i want that appointment to only go into the hotmail.com calendar

Then all his devices and appoints will be in one calendar and easily seen and maintained

How can i do this with IMAP accounts please