So here is how to add Yahoo Mail Folder in Outlook 2007.
Right click on the main folder (If you have multiple accounts, you can choose the account under which you want to have Yahoo Folder) under All Mail Items and Click New Folder. Give the name of your choice and click OK.
Right click on the new folder that you have created and choose Properties. Go to Home Page tab and write the web address into the address box. In this case, it would be http://mail.yahoo.com. Also check the box ‘Show home page by default for this folder‘.
Now click on this folder and you will be able to access Yahoo Mails within Outlook. If you have the new version of Yahoo Mail, you can also access Yahoo Messenger within Yahoo Mail.
How to change the home page or the startup page inside the Outlook?
You can also change the Yahoo Folder that you created now to the home page or the startup page of Outlook so that you will be first directed to Yahoo Mail as soon as you open Outlook.
To change the startup page, Click on Tools -> Options -> Other tab -> Advanced Options. Under general settings, click on Browse button and choose the folder that you created now and Click Ok.
Other Startup Pages: If you are using Gmail, Live Mail or other emails for which you have POP3 access, you may still want to use this feature for setting this as the other popular homepages like My Yahoo, iGoogle, My AOL, Live Homepage etc.
Met dank aan de originele poster: