[Dovecot] Mixing MBOX and Maildir?

Is it possible to have some folders be mbox and some be maildir format?

Why you might ask? Sometime for diagnostics I have the Exim MTA archive certain messages in MBOX format and then I want to copy them into an imao folder to look at them with Thunderbird. When I used MBOX all I had to do was copy the file into a folder and it worked. So what I'd like to do is have a folder with a special name that is MBOX and the rest be maildir.

Ideally have Dovecot be smart so that if the file exists then it is assumed to be MBOX and if the directory exists then it is assumed to be maildir. Has anyone done anything like this?