You should probably look at the docs here http://hudson.qa.jboss.com/hudson/job/jbpm/lastSuccessfulBuild/artifact/jbpm-distribution/target/jbpm-5.3.0-SNAPSHOT-docs-build/jbpm-docs/html_single/index.html#d0e3990 to try to define your own UserGroupCallback implementation. Adding some extra data inputs to a task which the engine does not understand does not make much sense.
Hope this help.