You meant storing "start date", "end date" and other informations of process instance directly into process variable is less useful because the values are stored in binary format which cannot be queried for reporting/display?
I think I know how to do it after checking https://github.com/droolsjbpm/jbpm/blob/master/jbpm-persistence-jpa/src/test/java/org/jbpm/persistence/session/VariablePersistenceStrategyTest.java