Yes, you can use a configured JNDI datasource that will take precedence over a configured URL/user/password combination.
If you open the activiti-app.properties where you configure your database, you will notice the following commented out section:
# JNDI CONFIG
# If uncommented, the datasource will be looked up using the configured JNDI name. # This will have preference over any datasource configuration done below that doesn't use JNDI # # Eg for JBoss: java:jboss/datasources/activitiDS # #datasource.jndi.name=jdbc/activitiDS
# Set whether the lookup occurs in a J2EE container, i.e. if the prefix "java:comp/env/" needs to be added if the JNDI # name doesn't already contain it. Default is "true". #datasource.jndi.resourceRef=true
Uncomment the datasource.jndi.name property ans set it to a properly configured container datasource.