To work with JSON type messages, DataPower first converts it to JSONX, a proprietary XML based format to represent JSON message, does transformation and converts back to JSON (using store:///jsonxtojson.xsl) as per the requirements.
To convert to JSONX, DataPower provides 2 different ways
1) Using HTTP Convert Query Action
2) Setting RequestType as JSON and use "__JSONASJSONX"
Important to notice that both of these methods works on processing rule. Convert Action works as processing action added on processing rule and __JSONASJSONX also work when it's a input for a processing action.
But I have come across requirements where you need JSONX in a XSL file. In that case, a reusable service can be designed using any of above method and can be called from XSL file. Here is how it's going to look like.