How is the WP user Name field populated?

Since the Membership registration process only requires Username and Email Address to subscribe, how can we require the Name field be populated? I know we can add Buddypress custom extended profile fields, but we would like the actual Wordpress user Name field to be populated on registration.

It does seem as if Membership expects that this would be populated, because the Communication features include the %usernicename% constant. But I just don't see a way to get this field into the sign-up workflow.

We do *not* have a coding budget for this project, so any adjustments would have to be made through back-end settings.

Thanks!

- Hope, developer for the BCIA (Baby Carrier Industry Alliance)