The sourcePath parameter adds additional sources to be documented. The subpackages parameter can be used in conjunction to add specific subpackages in the source path.
<project>
...
<reporting>
<plugins>
<plugin>
<groupId>net.sf.jfxdplugin</groupId>
<artifactId>maven-javafxdoc-plugin</artifactId>
<version>1.0</version>
<configuration>
<sourcePath>${project.build.directory}/src/main/other</sourcePath>
<subpackages>org.myorg.pkg1:org.myorg.pkg2</subpackages>
</configuration>
</plugin>
</plugins>
...
</reporting>
...
</project>