Lukasz, the common way for including custom work item handlers for jbpm/brms in version 5 is to place the libs into the jbpm-console-server/business-central-server war, inside WEB-INF/lib so the jbpm runtime engine can find it. Then you need to register them on the ksession and that is again done via files bundled in the *-server.war. Inside WEB-INF/classes/META-INF folder you should find CustomWorkItemHandlers.conf file, you need to add your custom work item handler there in following form:
"NAME OF THE WORK ITEM": new FullyQualifiedClassName()
if there are any entires already defined just add them as next element of the array. This is MVEL file and you should be able to use ksession as constructor argument if you need access to ksession inside your work item handler.
HTH