[Exim] supporting spool files AND directories

Hello,

I am trying to configure exim to handle user mailboxes of the form
"username-boxname" such that it can cope with cases where the user
mailbox is a file and cases when the user mailbox is a directory
in which messages are stored in separate files.

I can work out how to write the necessary transports to do the delivery,
but my problem is that I can't find a way of writing directors that
can distinguish the use of a directory rather than a file.

Has anyone worked out how to do this?

Thanks in advance,

Dave