Hi Michele,
do you have solved the problem? If not, maybe this will help you, else maybe you could help me :) (you could see my post here)...
I'm getting a similar error, in the same line, calling the same method... but in my case I'm loading a jpdl.xml and the class cast it's different, throwing a similar error:
java.lang.ClassCastException: org.jbpm.jpdl.internal.model.JpdlProcessDefinition cannot be cast to org.jbpm.pvm.internal.model.ProcessDefinitionImpl
To be more exact, in my code the cast is from JpdlProcessDefinition to org.jbpm.pvm.internal.model.ProcessDefinitionImpl, ubicated in a different package from yours. Your package doesn't exist in the jbpm.jar distributable of version 4.3 (I supose this package existed in the version you used).
Any help would be appreciated.