No direct answer to your question, but maybe some hints:

I assume that your IMAP server runs on Linux or some other Unix.

Many IMAP servers (can) use the same mbox format as Thunderbird. Simply copying the Thunderbird mbox files to the Unix server may be sufficient. At least, you could try that.

Some IMAP servers (can) use the maildir, maildir++, or imapdir formats. All three formats are very similar. Upgrading the maildir format to maildir++ or imapdir is trivial and usually happens automatically. There is a perl script named perfect_maildir to convert a mbox file to a maildir directory, and from my own experience, the name is justified.

If you have no direct access to the IMAP storage, or you have to live with a Windows Server, consider using a Linux (virtual) machine with a temporary IMAP server as an intermediate step. Depending on the temporary IMAP server, either just copy the mbox files or use perfect_maildir to create maildir directories. After that, use a tool like imapcopy to copy from the temporary IMAP server to the final IMAP server.

Alexander