For merging different PST files of Outlook while retaining their unique folder structures, there is the software called Kernel for PST Merge.” This tool provides a special Join feature for this purpose. It enables the user to join multiple PSTs into a single PST without having to copy individual mailbox folders to the PST file. It also handles ANSI and UNICODE PSTs, password protected PSTs, as well as selected folder merge. The user can select the destination PST as either new or existing.
Read More - https://www.nucleustechnologies.com/merge-pst/