Yes of course, that's why all the classes inside: https://github.com/krisv/jbpm/tree/master/jbpm-human-task/src/main/java/org/jbpm/userprofile
exist. They abstract the model of users and these classes can be extended to wrap your user/group models. Please take a look at the comments in the classes, those probably are the answer to your question.
Greetings!