You can override the username or the email in the $userData using the nsl_registration_user_data filter.
Example
Code
add_filter('nsl_registration_user_data', function($userData, $provider){
$userData['username']=$provider->getAuthUserData('first_name').'-'.$provider->getAuthUserData('last_name');
return $userData;
},10,2);Available since: 3.0.15