To import your Yahoo! Mail address book, export your file in Yahoo! Mail as a Microsoft Outlook CSV file. Note that even though you might see the option to export to vCard, do not choose the vCard format; the exported format is incompatible.
To export your Yahoo! Mail address book:
Open up your Yahoo! Mail.
Click the Contacts tab.
In the upper right corner, click the Import/Export link.
Scroll down until you see the Export section.
Click Export Now next to the phrase Microsoft Outlook.
In the dialog box that appears, select the Save button and click OK.
Depending on which browser or operating system you use, you might be able to choose the location where the file is saved, or the file might be saved to a "Downloads" folder automatically.
The exported file will have the name "Yahoo_ab.csv," in case you need to search for it.