Populate unique column with default value

Hey,

I'm updating the user table in our db to have a new column "username"
as follows.
ALTER TABLE usertable ADD COLUMN username varchar(64)
UPDATE usertable SET username='<extension of current row>' WHERE username ISNULL
ALTER TABLE usertable ALTER COLUMN username SET NOT NULL
ALTER TABLE usertable ADD CONSTRAINT usertable_username_key UNIQUE(username)

I want to essentially do what i have above but need to populate the
username field with some unique value so that the last ALTER will run.
I have another column in this table that is unique, an phone
extension, and am hoping to use this as a default username. Is there
an elegant way i can set the username to be the extension of the
current row or should i just write a little loop that goes threw and
populates my username with the users extension manually for each user
in my db and then run the last ALTER.

Thanks,

Jon.