function wpf_use_names_as_logins( $user_meta ) {

    $user_meta['nickname']   = strtolower( $user_meta['first_name'] . '.' . $user_meta['last_name'] );
    $user_meta['user_login'] = $user_meta['nickname'];

    return $user_meta;

}

add_filter( 'wpf_import_user', 'wpf_use_names_as_logins' );