How do I transfer/export Yahoo Mail to GMail without forwarding?

That is, I want to retain the original header detail (author, date) as originally sent to my Yahoo account, and move it over to GMail. If I mass forward the email, then the new header detail (my Yahoo address and the current date) will be displayed, which I don't want to happen.

How do I transfer/export Yahoo Mail to GMail without forwarding?
I haven't tried to do this yet. However, I guess you can enable the full header details in the yahoo mail settings, and then forward the emails to gmail as attachments, maybe it will work. Just an idea. I may try it right now and let you know if it works.