Document number is needed when trying to impersonate a user that already has an account

It's less a bug report than a question about how impersonating users works : We did a customed authorization handler with only the fields "name" and "document numer" which was used in previous version to create managed users. In the 0.12, we have the possibility to impersonate already existing users. The form includes also the field "Document number". How are we supposed to get this document number ? Thanks !