The only way I know is using system-wide folders Folders » Add mailbox to list... » Directory path. I always have a "on transit" Windows folder (i.e. directory) for this particular use. I avoid the use of Folders » Add mailbox to list... » Username unless I'm adamant of no naming coincidences in both users' Folders hierarchy tree or one or both HIERARCHY.PM files may get corrupt.

Using an accessory directory, copy/move messages to that directory, change users (close Pmail » reopen it with new user). While on new user, repeat the process of assigning the new mailbox pointing to the same Windows directory. After that, retrieve all messages from there. You can keep this transit always active with no harm. I have two of them to which all my users have access so moving bunches of messages between them is pretty slick and safe. I've been doing things this way since Pmail 3.12c days.

You can also do this using Public Folders but I never used this method before ever.

I'm aware that messages can be moved between mailboxes but Pmail also has provision for setting up moving a folder or tray from one mailbox to another but, on clicking the Move button (or drag and drop) it always seems to come up with the message "You have attempted to move a folder between two mailboxes and one or both of these mailboxes does not support that operation."

That message implies that, under certain conditions, it is possible to move folders between mailboxes: if the implication is correct what are those conditions?

Bob