Is it possible and it is also possible to use Active Directy. Search the forum more carefully and you'll find a post with a basic implementation made for Active Directory. Actually it is really easy.You implement a certain interface and you configure your implementation class in your jbpm configuration.
It is explained in the developers' documentation:
http://docs.jboss.org/jbpm/v4/devguide/html_single/#customizingtheidentitycomponent
HTH