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Gmail doesn't do this. But if the emails do have text in the message, it can do it.
If you are using the Gmail web interface the answer is no, it's not an available option. However, you can bypass this restriction by sending an email using a script or any other tool where you can control the headers and using the Gmail SMTP to deliver the message. Every programming language allows you to write a script to deliver an email. There's also a ...
You are looking for resending rather than autoforwarding. Gmail only offers autoforwarding. However, some downloadable email clients offer a resend feature. You can download your Gmail messages to any email client of your choice using POP or IMAP.
I agree with @Andrew - Gmail won't handle this on its own. However, you could use an intermediary running procmail1, preprocess the mail and add something to the subject, or change its sender, so that it can be filtered secondarily in Gmail.
That's not going to be possible from within Gmail. The only way I can think to do it is to use a third-party email client (like Thunderbird) to export the messages to text files, edit them, then re-import them back into the email client, then add them to the Gmail account.
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