In Jbpm Version 4 jpdl process definitions allows you to set a multiplicity paramter for joins. Is this also possible with jbpm 6 and bpmn2?
not really, bpmn2 defines different types of gateways (converging) that provide semantics on how the join will behave. Maybe complex gateways could be of some sort of equivalent but that's not supported by jbpm.
HTH