By adding an <extends> tag to your package.xml, you tell the world that your new package extends an old package in some way. The new package could supersede the old one, or give new functionality to it.

...
 <name>Package_Name2</name>
 <channel>pear.php.net</channel>
 <extends>Package_Name</extends>
 <summary>Any one-line summary</summary>
...