Hello, @hl_7.
Welcome to Rogers Community Forums! 😃
Thank you for posting your query in the Community; I believe you are trying to add your Yahoo mailbox to your Rogers email (webmail) account. Since Yahoo provides Rogers’ email, the option to add Yahoo email is not available.
Depending on the device, you can either use email clients like Outlook or Yahoo Mail app to add multiple mailboxes for various providers.
Cheers,
RogersMoin