I am not sure about your question entirely, but I doubt its related to webservices ?
For a good library that can convert your
java objects to json, you can have a look at flexjson :
http://flexjson.sourceforge.net/
As far as the design is concern, I personally would hate to see List<List<HashMap<String,List<String>>>> or similar Instead, you can create meaningful objects to represent your collection.
eg: List<Employee> and employee would hold List<Address> and address would hold some other list and similar data etc...