You can manually add new contacts to Yahoo Mail by using the "Add a new contact" feature at the bottom of the Contacts tab. You can also add contacts from an email by hovering over the recipient's ...

With Yahoo Mail, click on Options > Mail Accounts > Add. You’ll then be prompted to add your old email account.

In this guide, we show you the steps to bring your Yahoo Mail account to Outlook.com, including contacts and calendars.