by Emma Richardson.
I haven't found a way to do with user upload but with ldap and I presume external database sync, you can actually leave the email blank, forcing them to enter it on their first login.
You could however use a plugin like global message to popup a box if their email matches a certain format (e.g. @nomail.org) with a link to edit profile. I have been meaning to write the global message template for this because I have the exact same issue but haven't got there yet..!