[рони]

mrmammoth,

function doAdd(str) {
           var arr = JSON.parse(str);
            !arr['param3'] && (arr['param3'] = [])

                    arr['param3'].push({
                        subparam1: 'vol1',
                        subparam2: 'vol2'
                    });

            return JSON.stringify(arr);
}
alert(doAdd('{"param1":"value1","param2":"value2"}'))