You can use below parameter to migrate encrypted messages to a specific PST file:
When using above parameter, SSDM will create the following 2 pst files:
UserID.pst (non-encrypted items)
archive.pst (encrypted items)
If SSDM already sees a PST called archive.pst, then it will create a new PST called archive_1.pst, it will not use existing archive.pst (previously created by MNE) to migrate encrypted messages.
You can follow below Microsoft article to merge existing PST files: