Question about Outlook Express

I've switched someone to an IMAP account from pop after they started using another Mac. Outlook Mac 2011 on this older mac has been having problems opening occasionally and freezing on the "Identity.." while opening. The OS has issues itself and I've done all the typical maintenance efforts. I've managed to get it open sometimes by starting in safe boot and run DB repair utility so it's not totally corrupted. The identity file is 40 GB so problems are inevitable.
Since exporting the olm froze Outlook I resorted to creating an IMAP account and copying folders from the POP account into it. Recently alot of the emails disappeared from the server folders so I went back and repeated that process. I also copied each folder to a desktop folder making .mbox files of them as a backup. Upon importing these folders I noticed .mbox files don't retain subfolder emails and also lose attachments in the emails it does keep.
This causes a problem since some of the folders are no longer in the pop account to drag and drop to the IMAP. Yes I had to delete some regrettably due to the enormous amount of folders. No point in importing .mbox and .eml since they don't keep attachments.
I'm currently trying to export the OLM again from a backed up pop copy but don't have my hopes up. Just looking for advice on how to get these pop folders, sub-folders and emails back into the imap account with the attachments and email body in original condition. mbox and .eml have been useless.

Thanks