In jBPM5, processInstance is saved automatically when it reaches a wait-state. There is no way to persist it on-demand.
You can find more information about process persistence in the documentation manual:
http://hudson.jboss.org/hudson/job/jBPM5/lastStableBuild/artifact/jbpm-distribution/target/jbpm-5.1-SNAPSHOT-docs-build/jbpm-docs/html_single/index.html#d0e2517