It's not possible to create a custom WIH using the procedure described in this article:
http://mswiderski.blogspot.com/2018/04/jbpm-work-items-are-really-simple.html

Problems:
1. jbpm-workitems-core test-jar is not available in maven (7.23.0.Final-redhat-00003)
2. the resulting workitem is generated with the group id and version of the upstream project regardless the archetype configuration