I managed to add custom service tasks with the web-based editor (which is far more powerful than IDE-based ones), no need to spend time on extending palette in Eclipse anymore :)
http://docs.jboss.org/jbpm/v5.4/userguide/ch.designer.html#d0e4355
Having checked the time of last commit for Eclipse-based JBPM editor, i saw that JBPM is dropping IDE-based editor :|