Extending Zend_Db - zend-framework

Question

I apologize if my title is a bit misleading and it turns out that it's some other class under Zend_Db.
I use the following method of extracting data from a MSSQL:
// $_config contains information about how to connect to my MSSQL server
$config = new Zend_Config($_config);
$db = Zend_Db::factory($config->database);
$sql = "SELECT * FROM table1";
print_r($db->fetchAll($sql));
So far no problem and everything runs smooth :).
Now I need to run some more complex queries with multiple rowsets:
$sql2 = <<<EOD
DECLARE #test INT
SET #test = 42
SELECT * FROM table1 WHERE col1 = #test
SELECT col2 FROM table2 UNION
SELECT col3 FROM table2
SELECT * FROM table3
EOD;
print_r($db->fetchAll($sql2));
I hope you get the idea.
Using $db->fetchAll($sql2); will result in
Fatal error: Uncaught exception
'Zend_Db_Statement_Exception' with
message 'SQLSTATE[HY000]: General
error: 10038 Attempt to initiate a new
SQL Server operation with results
pending. [10038] (severity 7)
[(null)]' in
\Sacp026a\sebamweb$\prod\includes\Zend\Db\Statement\Pdo.php:234
The following function will return all the correct rowsets:
function sqlquery_multiple($zdb, $sql) {
$stmt = $zdb->query($sql);
$rowsets = array();
do {
if ($stmt->columnCount() > 0) {
$rowsets[] = $stmt->fetchAll(PDO::FETCH_ASSOC);
}
} while ($stmt->nextRowset());
return $rowsets;
}
print_r(sqlquery_multiple($db, $sql2));
Now my big question is:
How do I extend Zend_Db, so I can implement and use the function above as $db->fetchMulti($sql2); instead of sqlquery_multiple($db, $sql2)?
Thanks in advance :)
NB: It's worth mentioning that I'm using the ODBC-patch in order to be able to fetch multiple rowsets in the first place.