Sign into your Gmail account. Scroll up to the gear icon in the upper right corner of the page. Click the icon.
Select "Add Another Email Address" in the Send Mail prompt.
Type in the email address and click "Save." Repeat these steps for alternate addresses.
Sign into your Yahoo! account. Select "Options" from the top menu on the home page. Click the "Mail/More Options" prompt.
Scroll to the "Account" icon and click it. Select the "Yahoo! Mail" prompt.
Enter the alternate email address in the "From" portion of the text box. Click "Save." Repeat these steps as necessary for additional emails.