Yes. I saw in other posts that you can use one ksession for every process instance; and it's also in the documentation:
Sessions can be created based on a knowledge base and are used to execute processes and interact with the engine. You can create as many independent session as you need and creating a session is considered relatively lightweight. How many sessions you create is up to you. In general, most simple cases start out with creating one session that is then called from various places in your application. You could decide to create multiple sessions if for example you want to have multiple independent processing units (for example, if you want all processes from one customer to be completely independent from processes for another customer, you could create an independent session for each customer) or if you need multiple sessions for scalability reasons. If you don't know what to do, simply start by having one knowledge base that contains all your process definitions and create one session that you then use to execute all your processes.
I'm still a bit lost in jbpm architecture, and trying to understand all the concepts. But I thought (or guess :-/ ) that in the future I will need one ksession per process instance...