Auto generated sequential usernames (or another field)

So I am creating a community for a certain hobby and have pre-printed stickers for users to put on their vehicle. I am looking for a plugin that will auto-generate a user name in sequential numbers starting with a specified number (instead of letting the person registering to choose a username).
Alternatively, I might settle for creating a custom field that is sequentially autogenerated (no input from user) if the username field is out of the question

Any ideas??

Thank you!