A CSV file is a set of data lines, each value separated by a comma. The data for a group is more than that so I doubt very much that you can do it.
Having got all the individual members of your address book transferred is it not as simple as creating a new group in yahoo and then using drag and drop to populate it from your Yahoo address book? You shouldn't need to retype everything.
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.