I don't really understood your problem. A service task can be configured as as workitemdefinition. You can also define parameters for it. After that it will appear in the service task palette.
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-828362-21163/310-297/JBoss+Guvnor_2013-07-16_15-41-46.png
You need to register the workitemhandler in the session and implement it.
