I don't think that would be too difficult, although I would think that your users might want something more secure than their phone numbers as their password.
To edit the user accounts, as far as I know you just need to go into Setup >> Classes >> Users [Class Group] >> User [Class]
The user class has a special data type called 'User Account' though--which is not edittable in the admin interface as far as I can tell... and if you edit that I'm not sure what will break.
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