Thank you. Well, these exact steps I did already, following your first advice and yes, it works.
What I wanted to know - is there a way to set up these usernames in advance - ideally, I would import the list of accepted users from a CSV file using ImportUsers/Upload menu. So far, I don't see a way to do it through Import
What is even more inconvenient, after creating a new user (either by hand or by import), I can not do the steps you suggest to define the username before the user goes through the Register process.
Anyway, I can do it by hand or a script, not a big issue
Thanks again