There are no examples yet on how to implement extensions.
Koen would say, learn EMF, the eclipe plugin mechanisme, and look at what is done in the jBPM GPD plugin. That is the best example
you should be able to get started with the reference manual materials.
but if you like more the easy reading kind of stuff, there's a great new book on jbpm. you can find a reference on my blog http://processdevelopments.blogspot.com/