Timestamp format on row_to_json
There is no way to influence the format used by row_to_json.
You could define a view on your table and use
to_char(row_added_dttm, 'YYYY-MM-DD"T"HH24:MI:SS.MS"Z"')to format the timestamp as a string.
Then you can use row_to_json on that view to get your desired result.