WPMU Memberships Forcing a Specific Member Signup Name

Hello,

On our site, the client has about 250 existing members. They prefer a setup where each person has to RESIGN up annually in May. They do not want automatic billing. One thing they do want is the ability for people to sign up and pay each year using their existing ID NUMBER they have with the organization. Its something like an 8 digit number like "12345678". So for instance, someone goes to register and pay on the Memberships plugin, their username would HAVE to be that 8 digit number. I honestly don't know if that's possible or not? One thing I was thinking is is there a way to have say their user name be their email address or something, then they would put their ID NUMBER into a custom field of some sort, and then their password, and then this would create their account? Could they then be able to either login with their email address OR their ID NUMBER? Please let me know your thoughts, any suggestions on how best to achieve this would be greatly appreciated.

Best,

AD