You can use asynchronous workitems. You don't really have to implement specific workitem for script A and then another for script B, which is a valid approach but more cumbersome, just create a generic workitem that calls arbitrary java from any class and method passed as parameters.
Ok, I've already see this workitems, but we've no able to use it, How can attach the workitems to the entire workflow? Can you explain me how to set-up my action? There's some documentation about, or better, some example? I read a lot on the web and i design different process solution but no-one works. My doubt is that is that in JBPM there is no support explicit for parallelism.
Please help me, I really like JBPM and we want to use this technology but we need to be sure that we can do what we wish.
Parallelism can be achieved as long as you run your activities/workitems asynchronously. Within the discussions if you search you'll find plenty of discussions on related topics.
The documentation of jbpm is really great,
The engine itself is single threaded (the same stands with other popular engines) as it should. . Howerver i think that is not what you're really after, what you need is your activities to be executed in parallel so async workitems will do nicely your job.
http://docs.jboss.org/jbpm/v5.3/userguide/ch.core-basics.html#d0e2327 (also take a look in the asynchronous handlers section etc)