PST Merge to quickly join, add or merge multiple PST file into single PST file including calendars, contacts and address book and other email items. PST Merge allows to combine multiple PST files such as email folders, address book, contacts, journals, notes, calendars, tasks to one single PST file.

https://gallery.technet.microsoft.com/office/Merge-PST-Files-into-One-420741c3