In JBPM5 you can use Task Nodes to do exactly what are you looking for.
For each Task Node (aka Work Item) defined in your process you need to attach a handler. Is up to the handler to continue with the execution of the process or not.
I tried to explain the concepts behind asyncronous and syncronous work item handlers here: http://community.jboss.org/message/600693#600693
Please try to see if that post helps you.