You just have to configure it:
<project>
...
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-ejb-plugin</artifactId>
<version>2.5.1</version>
...
<configuration>
<archive>
<manifest>
<addClasspath>true</addClasspath>
</manifest>
</archive>
</configuration>
...
</plugin>
</plugins>
</build>
...
</project>
When used, the copy of the artifact in your project will have the classifier appended to its filename. This can be used to differentiate multiple artifacts.