Take a look at domain-specific work items. They are a special type of Task nodes that allow you model an invocation in the process as a custom node type and then register a handler, which is a simple Java class that can then invoke anything you like.
https://hudson.jboss.org/hudson/job/jBPM5/lastSuccessfulBuild/artifact/target/jbpm-5.0-SNAPSHOT-docs-build/jbpm-docs/html/ch06.html
Kris