Same problem here, I cannot add calendars of colleagues/rooms to emClient, which work on Outlook and Outlook Web, as EABonney described.
However, if I’m planning a new event and use the “Scheduling” option, I can see availability/busy statuses of the colleague’s calendar. So it seems that emClient can read out the information somehow, but it just cannot display it as a calendar, which Outlook and Outlook Web can do.
This would be a really great feature to have in emClient, too!