Did you try using the following configuration ? :
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-antrun-plugin</artifactId>
<version>1.7</version>
<executions>
<execution>
<phase>process-resources</phase>
<configuration>
<target name="runbuild">
<ant antfile="path/to/build.xml"/>
</target>
</configuration>
<goals>
<goal>run</goal>
</goals>
</execution>
</executions>
</plugin>