add_filter('rewrite_rules_array', 'mmp_rewrite_rules');
function mmp_rewrite_rules($rules) {
    $newRules  = array();

    $newRules['portfolio/strona/([0-9]+)/?$'] = 'index.php?post_type=portfolio_biura&strona=$matches[1]';
    $newRules['portfolio/(.+)/page/([0-9]+)/?$'] = 'index.php?post_type=portfolio_biura&kategorie=$matches[1]&strona=$matches[2]';
    $newRules['portfolio/(.+)/(.+)/?$'] = 'index.php?portfolio_biura=$matches[2]';

    return array_merge($newRules, $rules);
}