Hi there,

you can schedule a call back with a rep through your self serve account and they can add the line to the account for you, just mention that you have your own SIM card and provide them the digits on the back. They should be able to use that one instead of sending you a new one through the mail. I hope this helps

You can’t.  When you sign up they send you a new sim card always.  Just keep your other sim (assuming it has never been used before) as a back up