Add auto increment ID as username

[sylvainraye]

Regarding the Q&A done on Magento connect, the username could be incremental digits for membership for example, here is a quote of some discussion:

Thx for this module, it works as i should - i´m using it to add member id´s to existing and new members - but i´m missing a auto-increment feature.
So that if the last created member is 000123 - it would auto set the next to 000123 (if not already used)

I´m not using the field in frontend, but yes i should be un-editable by the user in frontend - and i´m not using the customer ID, as i needed to be able to import existing usernames (user nr/ member nr) in a specific format