How to aggregate local system mail in KMail?

I’d like to add /var/mail or /var/mail/$USER as a mailbox/folder in kmail. How might I go about this? By the way, I’d like to automate this process in a script, so I’d like to, if possible, use $USER rather than actual username of my user, but either is fine ultimately.

All that I know of this topic is from What is the "You have new mail" message in Linux/UNIX? - Super User

I’m AFK but I’m pretty sure Kmail supports maildir format and it’s already added by default when you first set up Kmail.

Perhaps that’s the problem. I am never prompted to complete a setup process when I launch kmail.

The relevant mailbox doesn’t appear to be automatically available.

If you go under Settings > Configure Kmail… > Accounts > Recieving you can modify the local mail account and point it to wherever you want. Note that I think this only supports maildir, not the old-style mbox.

Any idea of how to do so more specifically? I ask because I still don’t see any mail:

Whereas I should see at least one:

By the way, a cursory glance demonstrates that mbox is indeed supported, even though it’s irrelevant to this since kmail appears to believe that /var/spool/mail is a valid maildir directory.