If you mean copy all the emails sitting on the Bt server to the ymail server then one way is forwarding all your emails using your BT account to your Ymail account using webmail.
If you just want to retain a copy of the mails in a convenient place then the easiest way is to use an email client to copy them to your computer. Email clients can be set up to get your emails from both accounts.
You can also use an email client to forward the emails to your ymail account if you want to access them on the ymail server.
Please post again if what you want to achieve is different.