Is there a simple way to convert Qupath annotation file .qpdata into .json file ?
Not in general; .qpdata files contain information that isn’t a good fit for JSON (and could result in huge files).
However selected parts (e.g. individual annotations) may be converted to JSON. See https://qupath.readthedocs.io/en/latest/docs/scripting/overview.html#serialization-json for more information.