How do I add an extra source directory that will be used by the maven-jxr-plugin?

<project>
  ...
  <reporting>
    <plugins>
      ...
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jxr-plugin</artifactId>
        <configuration>
          ...
          <includes>
            <include>src/main/java/**/*.java</include>
            <include>src/main/java2/**/*.java</include>
          <includes>
          ...
        </configuration>
      </plugin>
    </plugins>
  ...
  </reporting>
  ...
</project>