The User Registration Solution Pack has been designed to work as it does now with the email address as the identifier and this is enforced throughout the files in the Solution Pack. Updating this core functionality would require extensive editing of the code within the Solution Pack. These edits would require making use of Security Assist and DataAssist at a minimum and CSS Form Builder or Validation Toolkit to handle the validations. It is possible to manually remove the validations for the email.
If you have these tools available and would like some additional insight on the changes that will need to be made please post back and let us know and we can start on the first part.
