Hi that's a good question, because the answer will basically afect your architecture.
You need to consider the Human Task component as a different service. You can expose it via WS if you need to interact with it directly, if you only want jbpm to interact with it, Mina or JMS are available. Creating an internal mechanism to do that communication over web services is not a difficult task but it's not there. I was working in a project called smart task which one of its characteristics is to expose the human tasks via web services using the WSDL provided by the HT-WS specification. You can take a look at that project if you really need to expose the human task server via WS, but I don't think that as a requirement. -> https://github.com/Salaboy/smart-tasks
Cheers