AFAICT a par file is just a zip/jar file containing your jpdl, jbpm.cfg.xml, database(hibernate) config next to class files you use in your process (DecisionHandler implementation for example).
An example of a bar/par file can be found in the examples/target folder of you Jbpm installation after running ant deploy.examples as mentioned in the user guide.
Johan is right. Also if you use the GPD3.1.x and do a 'save locally' you get a par file. It does not contain any deploymentdescriptors