If you need to put more stuff in your classpath when Surefire executes (e.g some funky resources or a container specific JAR), we normally recommend you add it to your classpath as a dependency. Consider deploying shared jars to a private remote repository for your organization.
But, if you must, you can use the additionalClasspathElements element to add custom resources/jars to your classpath. This will be treated as an absolute file system path, so you may want use ${basedir} or another property combined with a relative path. Note that additional classpath elements are added to the end of the classpath, so you cannot use these to override project dependencies or resources.
<project>
[...]
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.5</version>
<configuration>
<additionalClasspathElements>
<additionalClasspathElement>path/to/additional/resources</additionalClasspathElement>
<additionalClasspathElement>path/to/additional/jar</additionalClasspathElement>
</additionalClasspathElements>
</configuration>
</plugin>
</plugins>
</build>
[...]
</project>