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

Mark