Hi,
In order to update your site with external database you need to do or pay somebody to do the two following steps:
i. After studying the elements of the extrenal XML feed and your database structure a php file should be written which would parse the suitable elements and inject them to your database.
ii. A cronjob should be set at your cpanel which would make the above described php file executed periodically. The time can be every 5 minutes or what you prefer.
This way the database import from xml file is solved.
For further information you may want to read these resources:
http://www.kirupa.com/web/xml_php_parse_beginner.htm
http://ch2.php.net/simplexml
Regards
Zoli