After my post of yesterday about how to import Thunderbird Mailboxes to a Gmail account, today I've found the trick on how to import also the Outlook Mailboxes (PST format) to Gmail...
You can use this tool called Mail Navigator. This is a PST Reader and what you've to do is simply extract messages from the PST archives of MS Outlook, load the mail folders with MailNavigator, and then export the messages in MBX format (via the menu Message, Save As). After that, you can use the Gmail Loader tool.
The import is performed! 