The removal of the dash has nothing to do with Unicode usernames. That’s happening because the import script manipulates the username during the import.
I don’t think there’s any need for that. Try replacing those lines with the following code snippet. It should work.
@users[u['id'].to_i] = { id: u['id'].to_i, username: u['username'], email: u['email'], created_at: u['registerDate'] }