I'm not sure you can actually do that. Looking at the API documentation (http://www.cocos2d-x.org/reference/native-cpp/V3.0alpha0/d7/d76/classcocos2d_1_1_sprite_frame_cache.html#a76145a0afd578220bb0973be4e7f5e9d) it states
Adds multiple Sprite Frames from a plist file.
Assuming I'm right (still to be proven!), you can convert (under OSX) from a json to a plist using command line:
plutil -convert json Data.plist -o Data.json
Or you could edit cocos2dx CCSpriteFrameCache.cpp method: addSpriteFramesWithDictionary in order to parse .json maybe.
Let me know if this helps, or if you find a way to load directly from json in a smarter way I'm also interested in this!
why do you need json instead of plist?