In my opinion it is not possible to achieve what you described, at least not exactly. In general, process definition (bpmn2 file) is read only on runtime but there could be two options that could be interesting for you:
- ad hoc process
- use multiple instance characteristic of an activity
Ad hoc process allows you to define activities that are not connected (or does not have to be connected) to create a flow of activities and activities that should be invoked at runtime can be selected per process instance basis. More information can be found at jBPM docs
Multiple instance characteristic gives you an option to decide on runtime how many instances of an activity should be created, in your case how many human tasks - mote that in this scenario it will create multiple instances of the same definition of an activity.
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If you want to make a change while the process is in use and if the change is permanent and global not per case/user or based on some logic, you can easily achieve that by using Guvnor repository and its designer. I think you may also do something like that programmaticaly and via the REST API of Guvnor as well as compilation of the package etc