I believe, CesiumJS is not supporting JSON format. I may be wrong!
The 3D models are designed in Blender and exported as Collada (for Cesium and converted into gltf) and JSON (for Three.JS).
Three.JS is having separate plugin for Blender to convert to model into JSON (https://github.com/mrdoob/three.js/tree/master/utils/exporters/blender).
If CesiumJS supports same JSON format, then we can use it directly. But I didn’t see a single example with JSON loader in CesiumJS.
Regarding geoposition, I try to convert the XY to LAT & LONG from this link (http://www.whoi.edu/marine/ndsf/cgi-bin/NDSFutility.cgi?form=0&from=XY&to=LatLon). But it’s of no use!