-> https://github.com/droolsjbpm/jbpm/blob/master/jbpm-human-task/jbpm-human-task-core/src/main/java/org/jbpm/task/admin/TasksAdmin.java
If you don't have that interface, it means that you are with an old version of jBPM, and you will probably need to implement something like that.
Cheers