Don't sign up online, instead call them (customer service is 1-800-444-3333, they can probably transfer you to new accounts from there). A friendly representative will find the best plan for you with the highest amount of miles. Tell them that a friend of yours is receiving 10000 miles and start negotiating from there.

PS: Same as with AA customer service, if you don't like what you are hearing, thank the person, hang up and call again.