This is the de facto example for sar packaging. This example pom.xml demonstrates what you need in order to have maven generate a sar file using maven-sar-plugin. The <extensions/> element must be present and must be set to true for the plugin to operate correctly. Also remember to set <packaging/> to sar.
<project>
...
<build>
...
<plugins>
...
<plugin>
<groupId>net.sf.maven-sar</groupId>
<artifactId>maven-sar-plugin</artifactId>
<version>1.0</version>
<extensions>true</extensions>
</plugin>
...
</plugins>
...
</build>
...
<packaging>sar</packaging>
...
</project>