jbpm libraries consist of multiple .jar files. So it's possible that you forgot to add drools-compiler artifact in particular, which contains class KnowledgeBuilderFactoryServiceImpl.
But I still think that better solution is to include jbpm .jar files as a part of your module. I mean to have them included by build inside of deployed .war or .ear files/directories.
Before I tried to put the dependencies as modules, I' ve tried to include the jars inside the ejb module, but they are not included on the jar. (I use maven in my project). Then I tried to put that depencies on the ear, but the problem still persist. Finally, I deployed the jbpm-gwt-console-server.war and jbpm-human-task.war and put those as dependecies of my project and it works!. But that it not the best approach. Maybe do you know how to include those dependencies in order to make them available to the ejb module?
Thanks in advance.