problems copying email folders from one to other mail account on same imac

I have 2 log in accounts on the same imac, and have been merging them. To complete, I only need to copy over all Mail mailboxes from the old user mail account to my current log in mail account before I finally delete the old user account. BTW, both accounts are identical POP mailboxes with the same email address.

For Mail I repeatedly Export and Import, using .mbox OR Mail format. The Export takes place, the Import steps through the process (I even see the sub folders as they list in the Import window) , but I always end up with an imported new mailbox with no content.

can you tell me how the copying over should be done, please?

Qickrush