not entirely sure if that is what is required but you could start with something like this:
<task name="review" >
<assignment-handler class="org.jbpm.examples.task.assignmenthandler.AssignTask">
<field name="assignee">
<string value="johndoe" />
</field>
</assignment-handler>
<transition to="wait" />
</task>
Please take a look at user guide for more details