How to Use/Acquire Custom ID in WPMU Memberships

Hi there,

I asked this question previously but it has gone unanswered for some time. My question is that I am building a Membership site for the Pennsylvania Association of Registered Nurses & Dieticians ("PAND"), a state licensed professional association. When one of the nurses becomes licensed, they receive a license ID number, usually an 8 digit number like "12345678" from the state. Once they are licensed by the state, they have the option to buy a membership to the organization, similar to the way how Attorneys are licensed by the state, but they have the option to join professional organizations, such as the American Bar Association or the PA Bar Association, and pay yearly dues.

The way the client works now is that each year the person must pay at the beginning of May their yearly dues.

The client wishes, if possible, that when someone signs up/registers for the current years membership and pays their current years annual dues, that they do so using their state license ID#. Ideally, this would be the situation where a nurse goes to their website, registers for the 2014 year, and pays their dues, but registers using their state license ID #, instead of an arbitrary user name or their email.

Is there anyway to force a registering member to use their ID number to register and become a member of the site? Or if not, is there a way to add a custom form or something where they sign up with their email address or user name, add their password, and then also add their Member ID? Just to keep track of it? FYI, the only thing the members do once registered is they are given access to a protected content page containing various PDFs and what not of "members only" info.

Thanks very much,