Please read the AntRun plugin usage guide first.
Here's an example configuration - you'll get the general idea.
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>xdoclet-maven-plugin</artifactId>
<version>1.0</version>
<executions>
<execution>
<phase>generate-sources</phase>
<goals>
<goal>xdoclet</goal>
</goals>
<configuration>
<tasks>
<ejbdoclet destdir="${project.build.outputDirectory}">
<fileset dir="${project.build.sourceDirectory}" includes="**/*Bean.java"/>
<entitycmp destDir="${project.build.outputDirectory}"/>
<deploymentdescriptor
destDir="${project.build.outputDirectory}/META-INF"
/>
</ejbdoclet>
</tasks>
</configuration>
</execution>
</executions>
</plugin>
As you can see, I've also specified destDir attributes on the subtasks (like entitycmp). This is because of a bug in XDoclet, when XDoclet is used more than once (for instance, in a reactor build).
The goal to be called is xdoclet:xdoclet.