What I assume you are talking about in importing or copying folders full of emails from a local pop storage location to a remote IMAP server. This can be done, but the process is not reliable, silent timeout occur and the mail being processed can simply disappear.

I suggest you rethink the process you wish to follow. Perhaps just relocate the folders to the "Local Folders" account. That is a local copy on your machine which you will need to accept responsability for backing up. But it will not potentially loose your mail.

I am just trying to change from accessing via POP3 to IMAP or Exhange based access. FXDomains, my email provider switched my email servers to Microsoft Office 365 based ones. They now no longer support POP3. All of my email accounts where setup on POP3. I can no longer access them in my POP3 based Thunderbird accounts. Thunderbird does not allow me to change the from POP3 to IMAP as the access method. I setup a new account on my PC with IMAP to maintain access to my emails. But, I do not have subfolders and filter rules. FXDomains suggested OWL for exchange. I setup it up but it does not allow two emails with the same name. IMAP tags IMAP on to the current email address. My only plan now is to find away to backup my emails, subfolders, and filter rules. Then create a new setup with OWL access email account and then import the backup files. I do not know any other way, since Thunderbird does not allow changing from POP3 to IMAP. I just want to be able to keep my current local Thunderbird setup for reading emails. I love Thunderbird.