I believe you sign in to the btopenworld broadband website and manage your email addresses in there. You simply enter your primary email details to get in.
Just a question though. If you are a business, you could register a domain name in your company name and have unlimited pop3 email accounts such as [email protected]. or .com or .net whichever domain name you register. You only use BT to SEND emails out. In outlook you tell it the reply to email address is your domain one. This looks far more professional for a company.