I'm not sure why you think that Windows 8 Metro Mail App does not support BT Yahoo email. I have my BTinternet email as an account on Windows 8 and it works fine. I set it up as a POP3 account with the following settings:
Incoming - winmetro.imap.mail.yahoo Port 993
Outgoing winmetro.smtp.mail.yahoo Port 465.
Both incoming and outgoing servers require SSL ticked
Outgoing server needs authentication and both the use same username and password for send and receive mail.