Try this configuration. It worked for me:
<plugin>
<artifactId>maven-war-plugin</artifactId>
<version>2.4</version>
<configuration>
<warSourceDirectory>mywebRoot</warSourceDirectory>
<warSourceExcludes>source\**,build\**,dist\**,WEB-INF\lib\*,
WEB-INF\classes\**,build.*
</warSourceExcludes>
<webXml>myproject/source/deploiement/web.xml</webXml>
<webResources>
<resource>
<directory>mywebRoot/WEB-INF/lib</directory>
<targetPath>WEB-INF/lib</targetPath>
<includes>
<include>mySystemJar1.jar.jar</include>
<include>mySystemJar2.jar</include>
</includes>
</resource>
</webResources>
</configuration>
</plugin>