You have to do the conversoin manually or in the query (like jBPM does). This is the example of the 'TasksAssignedAsPotentialOwner' query:
select
new org.jbpm.task.query.TaskSummary(
t.id,
t.taskData.processInstanceId,
name.text,
subject.text,
description.text,
t.taskData.status,
t.priority,
t.taskData.skipable,
actualOwner,
createdBy,
t.taskData.createdOn,
t.taskData.activationTime,
t.taskData.expirationTime,
t.taskData.processId,
t.taskData.processSessionId)
from
Task t
left join t.taskData.createdBy as createdBy
left join t.taskData.actualOwner as actualOwner
left join t.subjects as subject
left join t.descriptions as description
left join t.names as name,
OrganizationalEntity potentialOwners
where ...