You can easily add multiple PST files at Once in MS-Outlook with the best Outlook PST Merge Tool is KDETools PST Merge. It easily Joins multiple items such as Emails, Calendars, Tasks, Notes and all without duplicates PST files. During Conversation, it easily removes the duplicates PST files. Merge PST files ANSI and also UNICODE formats. It can add multiple PST files with selected date ranges according to needs of users. Merge PST items without installing of MS-Outlook. Free Version also free to download and easy to use.
More Info: https://www.kdetools.com/pst-merge.html
Combine Outlook files with all data or selected data with Merge PST Tool in just 4 simple steps. I have used this application a week ago and got fine results therefore, I would like to suggest you to use this application to merge Outlook data files into one PST file. It help you to join PST files without Outlook installation.
Visit here for more info: https://www.osttopstapp.com/pst-merge.html