This is exactly what you need.
You can define multiple database connections in your config.xml
<!-- osCommerce db/read/write -->
<oscommercedb>
<connection>
<host>localhost</host>
<username>root</username>
<password>pass</password>
<dbname>oscommerce_database_name</dbname>
<model>mysql4</model>
<initstatements>SET NAMES utf8</initstatements>
<type>pdo_mysql</type>
<active>1</active>
</connection>
</oscommercedb>
<oscommercedb_write>
<connection>
<use>oscommercedb</use>
</connection>
</oscommercedb_write>
<oscommercedb_read>
<connection>
<use>oscommercedb</use>
</connection>
</oscommercedb_read>
<!-- end osCommerce db -->
and then simply call the resource like this
$read = Mage::getSingleton('core/resource')->getConnection('oscommercedb');
In the original answr you will find some more info, but it is possible at least.