I read the cpanel link that was left in this discussion. The commands are used on a unix server for copying emails from one directory to another. If the person is an office-person and does not know unix shell or basic unix commands, it will be difficult for them to grasp the concept.
So I know unix, but do not have server-level access right now (not a SysAdmin) and am having sort of the same issue.
I REALLY NEED Roundcube to supply either an external app that we can load on our client or better yet a selection off of the MORE... menu pick that allows for Bulk Download of EMails from a folder and produces a .mbox file. Then we can use Roundcube's already-supplied Import facility. Reason why we need this where we are is that we are switching accounts to support a re-branding project for our museum.
Hopefully the RoundCube folks will read this and make this change.