How to Convert Outlook OST to Zimbra TGZ Format?

I have some old Outlook OST files that I need to access in Zimbra. Since Zimbra uses the TGZ format, I am seeking a reliable method to convert OST files to TGZ format without losing any emails, attachments, or folder structure. I tried searching for manual methods, but I could not find a proper solution. Has anyone successfully done this migration? Can you please suggest the best method or tool to convert OST to TGZ safely?