You cannot transfer email addresses with Thunderbird. Thunderbird is an email client you use to retrieve and send mail via an email account you've established with an email provider, e.g. BT.

Are you talking about setting up other accounts in Thunderbird, i.e. accounts you have with an email provider other than BT?

In that case simply run the Thunderbird new account wizard. https://support.mozilla.org/en-US/kb/automatic-account-configuration

Do you mean contacts? In an email forum like this, 'address' has a particular meaning: it's the character string like '[email protected]' to which the email is sent. When you talk about wanting email 'addresses' from your BT Mail, I assume that you actually mean the saved details of people you have written to: not just their email addresses but also their names and any other information you have stored. It's usual to refer to such data as 'contacts' information. If that is what you mean then you need to find out whether BT Mail allows you to export the data and in what format. Then post back here and we can try to help you import it into Thunderbird.