You can save your address book as a text file, (Address Book > File > Export, and in the menu that appears select "Other address book", then save as a text file, comma separated values). Save it as a .txt file onto your desktop, or similar temporary location.
But it all depends whether Yahoo can accept importing a text file. Even if it does, the formatting may not be the same. However I see from the Yahoo site Address Book importer that it does accept importing from a text file, (comma separated values).
But I would suggest you see if you can "save" Yahoo's address book first in any way, in case it doesn't work properly.
I hope this helps
I am using outlook express 6.0, windows XP on my computer. I want to import my Outlook address book into Yahoo but every time I save the address book as a csv file and then try to import into Yahoo it says that my file is not a valid csv file. Am I somehow saving it incorrectly. Thanks for any help. I have followed the directions on Yahoo exactly but to no avail.