To Backup IMAP mailboxes to PST format? Then I would like to suggest you can also try Mailsbackup IMAP Backup Tool to backup IMAP mailbox emails to local PC as PST, MBOX, EML, PDF, etc. IMAP Backup and Restore tool enables to backup IMAP Server and IMAP account to IMAP, Gmail, Office 365, etc. You can also save IMAP account emails to various desktop based email clients such as Thunderbird, Outlook, Lotus Notes, Windows Live Mail, Zimbra, etc. 

Check for more details at: https://www.mailsbackup.com/imap/