There is need to extend the current jBPM simulation backend and Designer to include KPI to the current simulation capabilities. This is something we are going to work on and I think what you are doing would fit there. If you want ping us on IRC and we can see if what you doing can be incorporated into the community code.
To answer your question, you would have to add your custom plugin to designer code ( https://github.com/droolsjbpm/jbpm-designer ). You can see a lot of examples here https://github.com/droolsjbpm/jbpm-designer/tree/master/jbpm-designer-client/src/main/resources/org/jbpm/designer/public/js/Plugins. You register them in plugins.xml and also include them in inlineeditor.jsp. Hope this is a good start, if you have more questions ping me on IRC and I'd be glad to help.