On Fri, May 25 2018, Xavier Maillard wrote:
> I should have asked something more more emacs-ish :) (eventually
> even
> better: a mu4e-centric solution).
Since mu4e doesn't read mbox, the latter is unlikely. :-) But Gnus
can handle both mbox and maildir, so in theory you could set up
Gnus and then use it to copy the messages in your maildir folder
to a newly created mbox folder. (I've done this in the other
direction with mutt.) I suspect a non-Emacs solution will be
quicker and more convenient, though. :-)