...

In case you are unwilling to remove jars from your current environment. May be you want to use your version of jars and geronimo ships a very different version of jars. In that case also geronimo provides an easy configuration. You can configure geronimo-web.xml to add <inverse-classloading/> tag. Using this tag the classloading hierarchy will change to Child First.

JSP Reloading

If you have configured Tomcat for JSP reloading than you would want to have the same in geronimo. This can be easily achieved in geronimo by editing <geronimo_home>/var/catalina/conf/web.xml.

Search for <servlet-class>org.apache.jasper.servlet.JspServlet </servlet-class> and add the following elements under the servlet class

<init-param>
<param-name>development</param-name>
<param-value>true</param-value>
</init-param>

<init-param>
<param-name>modificationTestInterval</param-name>
<param-value>10</param-value>
</init-param>

References

Following resources can be useful while migrating from Tomcat to Geronimo
1) http://geronimo.apache.org/schemas-2.1/docs/geronimo-module-1.2.xsd.html- Look for inverse-classloading. You may also want to know more on dependencies, hidden-classes, non-overridable-classes. These all elements will be helpful in modifying the geronimo-web.xml.
2) Learn how to configure JMS with apache geronimo - http://cwiki.apache.org/GMOxDOC21/web-application-for-jms-access.html
3) Learn how to configure JDBC with apache geronimo- http://cwiki.apache.org/GMOxDOC21/web-application-for-jdbc-access.html
4) For security related configurations you can refer- http://cwiki.apache.org/GMOxDOC21/configuring-security.html
5) More on configuring various services- JMS, JDBC http://cwiki.apache.org/GMOxDOC21/configuring-services.html