better questions maybe?

[Erik Quaeghebeur]

> […] kmail has created new,cur & tmp subdirs in /var/mail.
> I can have procmail put individual mail files in one of those 
> dirs, but which one?

https://en.wikipedia.org/wiki/Maildir#Technical_operation

AFAIU: First write to tmp with filename as suggested, then move to new.

Others have described how to do that with procmail: 
https://wiki.dovecot.org/procmail, 
https://wiki.debian.org/MaildirConfiguration.