I think it would be supported in next version, and it is currently in master. It is a callback interface where you can integrate with your Identity Management System. I did not try it yet:
https://issues.jboss.org/browse/JBPM-3276
When I had to make it, I used the methods that bpmn2user suggested to make mantain human task tables and my identity management tables in sync.
The users and passwords you saw I think they are from jbpm console. But in Human Tasks tables, you don't store passwords, only users and groups ids.
Regards,
Demian