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since you're running in domain mode then access to war file is rather limited so you might want to copy files that are inside the war/WEB-INF/classes/META-INF/*.conf into home directory of the user that runs JBoss server and thee files should be discovered on restart and loaded. Next define your handlers in these files in home directory and that should give the possibility to use custom handlers. Though any change will require server restart.
Ok, the problem was that in WorkItemDefinitions.conf I just pasted what was in  link and just append my handler. The file parser in jBPM couldn't resolve the second handler as the mapped class was not present on classpath and the parser stopped parsing there, so it did not get my handler mapping. I just left one line with my work item handler mapping and it worked. So the step by step guide in  is correct. Thanks Maciej for help though.