From what I've found elsewhere, it looks as though setting your language to Greek will automatically apply « » quotation marks.
http://apple.stackexchange.com/quest...quotes-in-word
It seems there is also a keyboard shortcut (depending on your keyboard) that might be a little less cumbersome than the Alt+ method:
Quote:
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With a US International Keyboard and corresponding layout, Alt Gr+[ and Alt Gr+] can also be used.
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Good luck!