Combining Outllook in the new institutional set-up with my use of PM is hopeless. There have been discussions here about moving mail to Outlook, but I have the opposite question. I'm trying to move true correspondence out of my institutional email, but some will, of course, still land there. I can export individual emails to .pst; is there a way to incorporate them into Pmail? (Forwarding them individually is possible, but it creates a new email from myself to myself)

This used to be easy. You could make an IMAP connection to the Outlook mailbox with Pegasus Mail and then copy the messages to a local Pegasus Mail folder. That was before the enforcement of OAUTH2 by Microsoft. Pegasus Mail can not do Microsoft OAUTH2. I understand that institutions can choose to allow connection by means other than OAUTH2 so checking to see what their IMAP connection credentialing requirements are may be worthwhile.

The implementation of the Outlook system here as made it inaccessible from any route that does not go through the exchange(?) set-up. Bluemail does, but I don't see how that would help me move the mail into PM. So the question is about individual emails, which can be exported as .pst files. Can the .pst file be converted or imported?

Here's what actually was available in 2012, the Wayback machine even has it listed but you can't get it anymore according to Martin's most recent list which you can get via this post which only contains these tools:

Bearhtml 4.9    Bearhtml viewer
BearSupport     BearSupport files
FolderREP 1.6   PM folder reports
MapiPM 1.8      Outlook email interface
Pmadrcsv 1.5    PM addr book export
Pmical 3.2      ICALENDAR Event display
PmVcard 1.2     Display business cards
Virscan 5.2     PM Virus checker
IEFBR14         Disable Virscan for MS Virus checking
Virprot 2.0     Mercury virus checker
SendDoc1 2.1    OpenOffice and LibreOffice support (2 part)
SendDoc2 2.1    OpenOffice and LibreOffice interface
Rssmail         RSS to Pegasus Mail

Sorry ...

Setting Outlook to forward all mail should work for all future messages but I don't know of a way to get existing messages into Pegasus Mail if IMAP is not an option.

IIRC Pegasus Mail can deal with MBOX file type folders, and if still so here's a page pointing to solutions orginally figured out by Thunderbird users: convert pst to mbox and import mbox to thunderbird. There's a commercial page out there of a company selling converters called "pcvare" (for obvious reasons I don't link to it here) which provides screen shots suggesting to just place MBOX files into your mailbox file directory (such as C:\PMAIL\MAIL\ADMIN) for making Pegasus Mail see them (search for "thunderbird to pegasus" ).

PS: It's recommended to convert it (i.e. move its contents) to a Pegasus Mail folder for faster access and enhanced data integrity.