Great news here! If you're already getting mail from an external account delivered in your Yahoo! Mail account, then you're already set up to send mail from that address, too.

How? Just click the down-arrow at the end of the "From:" box at the top of a Compose page. From the pull-down, click the address you want, and that will be the one that appears for anyone who's receiving this email. It's also the address to which their reply will be sent.