<project>
  ...
  <build>
    ...
    <plugins>
      ...
      <plugin>
        <groupId>org.apache.tomcat.maven</groupId>
        <artifactId>tomcat6-maven-plugin</artifactId>
        <version>2.1</version>
        <configuration>
          <systemProperties>
            <example.value.1>alpha</example.value.1>
            <example.value.2>beta</example.value.2>
          </systemProperties>
        </configuration>
      </plugin>
      ...
    </plugins>
    ...
  </build>
  ...
</project>