Think of Tomcat as a servlet container. The servlet is defined to Tomcat in the application's WEB-INF/web.xml file. Following is an example of defining servlet servSomething to the Tomcat containing the Indus FI Connect product and allowing it to use FI Connect's configuration. This is just an example of plunking a servlet into a pre-existing product. All of this stuff is standard Apache/Tomcat/Java and can be found described on Sun's websites.
<servlet>
<servlet-name>servSomething</servlet-name>
<servlet-class>icservlets.servSomething</servlet-class>
<init-param>
<param-name>ConfigFile</param-name>
<param-value>docs/PPSAP.properties</param-value>
<description>The path to the configuration file for Indus FI Connect.</description>
</init-param>
</servlet>