You can work around this if the mailboxes are created.
I find it strange that your cpanel uses mbox as it is an ancient technology and as far as I know cpanel advises that mboxes should be converted to maildirs.
Anyway ..
I don't know the errors , but I would make the following approach:
get a mb2md and convert the mailboxes to maildirs
get those maildirs from cpanel and simply copy to the qmail folders.
Possibly you'll have problems with viewing horde , but if you subscribe the sub-folders in horde that should be solved.