To import PST files to Shared Mailbox in Office 365, you should try Yota PST Converter. The software allows you to migrate all your Outlook data, including emails, contacts, attachments, and calendars, in just a few clicks to your desired folder. You can upload large PST files at once without losing anything. It also supports selective import, so that you pick only the folders you want. The process is quick, safe, and doesn't require Outlook installation. The tool preserves your email formatting, metadata, and original folder hierarchy. Free demo is also given to test the working of the tool.