This is what the tech person emailed me:
Once you have activated your account you will have the option to change the portion of your email address which is before the @ symbol. To do so you will have to go to snap.it.northwestern.edu and click on the link that says 'Change Northwestern e-mail address'. If you need help with this once your account is active please write back and we will assist you further.
For a long time, my NetID wasn't working. Then I just tried it one day (yesterday) and it worked. I didn't do anything different.
At any rate, once you have your NetID, you can activate your email and do what the guy said above to change your "alias." Then you will technically have two emails; all the email sent to
whateveryouwant@northwestern.edu will be sent to the ugly email address (lol).
No, I didn't pay my tuition deposit yet! Too soon for that stuff, haha. And regardless, I haven't received my financial aid info yet.