If you want some custom code you should implement that in work item handlers. Then - repack gwt-console-war with dependencies (if you need them for ws invocation ), and use task node in process witch will be bound by name of work item handler. For work item handlers refer to jbpm 5 documentation:
http://docs.jboss.org/jbpm/v5.2/userguide/ch13.html#d0e3937
http://docs.jboss.org/jbpm/v5.2/userguide/ch13.html#d0e3926
p.s. simple way to repack:
copy dependencies to jbpm-installer/db/driver
edit jbpm-installer/conf/META-INFO/drools.session.conf
run "ant install.jBPM-gwt-console.into.jboss" in jbpm-installer dir