I think the easiest approach for doing this inside Pentaho Report Designer is to use a Pentaho Data Integration transformation as a datasource. You simply create a KTR that calls the REST endpoint you want; JSON Input Step to extract the answers, and shape up the results so they are in a table like format and run it into a dummy step called "output". Then you copy that ktr file next to your prpt report and create a datasource pointing at the file and particular output step and viola. You are off to the races.