There is no standard JSON format for XACML policies, as already answered. Yet there are well-known conventions to convert XML to JSON (with limitations), mostly used by REST API frameworks. So if you know the XML format, the convention tells you the JSON format. For example, Apache CXF used to support two conventions: Badgerfish and the mapped convention. Badgerfish is no longer maintained in CXF therefore the mapped convention is preferred now.
The mapped convention is what AuthzForce Server - another ABAC/XACML implementation - uses for the RESTful PAP (Policy Administration Point) API, so that you can manage XACML policies in either XML (standard XACML) or JSON format. We used the JSON format for Javascript-based apps (e.g. web user interface) in particular.