You could use an add-on to re-post them to yourself.

https://addons.mozilla.org/en-GB/thunderbird/addon/mailredirect/

Better would be, if possible, to set it up as an IMAP-connected account, then you just copy the messages into the Inbox on the IMAP-connected account, sit back and wait for them to be transferred back to the server via the synchronization process. After use, you could remove the IMAP-connected account and revert to POP.

Do you choose not to use IMAP, or is it not available to you?

Thank you for the fast response. POP was a choice or rather tradition. Yes, IMAP is avaliable. Can I change an existing POP account to IMAP easily without risking the mails?

Pop is set up separately from IMAP, you cannot just swap.

Pop emails are stored on the computer in your Thunderbird profile folder. Usually, they are auto deleted off the server when downloaded, but settings can be set to keep copy on the server. You do have to be aware that this could easily fill up your server quota if not managed.

Imap mail accounts see a remote view of the folders on the server. You subscribe to see folders. Headers are downloaded and when you select to read an email it is retrieved from the server and stored in a temporary cache to facilitate viewing. You can synchronise subscribed folders for offline use to get a copy of the folder in your Thunderbird profile, but these folders are always being synchronised with server folders, so cannot be regarded as a backup. Anything you delete via webmail or phone from server will delete the email from thunderbird view as well because in effect, the emails are on the server and you would be using different methds of accessing the same folders. You can synchronise subscribed folders, go into offline mode and use 'Copy to' to get copies in the 'Local Folders' mail account. These emails are on your computer and do not synchronise with server. Very useful if you need to gain space on the server, but do not want to delete emails.

So I would advise you make backups on regular basis if using imap.

Zenos's comment on creating a new exisitng mail account and set it up as imap so that you can use 'copy-to' option to put emails back onto server is a good idea.

Thank you, for your detailed explanation. I will swich to IMAP.