To set a grouping for different packages, use the <groups/> configuration in your pom:
<project>
...
<reporting>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<configuration>
...
<groups>
<group>
<title>Core Packages</title>
<packages>org.apache.maven.plugin.javadoc.test1</packages>
</group>
<group>
<title>Extension Packages</title>
<packages>org.apache.maven.plugin.javadoc.test2</packages>
</group>
</groups>
...
</configuration>
</plugin>
</plugins>
...
</reporting>
...
</project>