For dynamic process definition modification, jBPM5 allows updating a process instance to a newer process definition.
For dynamically changing a process instance, it allows you to add new node instances (not part of the process definition) when using an adhoc (sub)process. By dynamically adding rules to the knowledge base, these rules could also be used to alter / override the default process definition behavior. Do you have any more concrete description of what you are trying to achieve?
Kris