This is the sample code for using RESTTask in WSO2 BPS. You can use WSO2 BPS to make REST calls within your BPMN processes. More information can be found in the BLOG post .
You can import this project to Eclipse BPMN designer and make changes. A deployment artifact is alredy available in the "deployments" folder. Just download this artifact and deploy it in WSO2 BPS. Start a new process instance from BPMN-explorer in WSO2 BPS. It will make a REST invokation and the result will be displayed in a User task. Refer  for more info on using BPMN with WSO2 BPS.