Например, вот так:
var list = {
"FailedCount": [{
"FailedCount_MEAS_VALUE": 1,
"DATETIME_CURRENT": "12:01"
}, {
"FailedCount_MEAS_VALUE": 0,
"DATETIME_CURRENT": "12:02"
}, {
"FailedCount_MEAS_VALUE": 3,
"DATETIME_CURRENT": "12:03"
}, {
"FailedCount_MEAS_VALUE": 4,
"DATETIME_CURRENT": "12:04"
}],
"SucceededCount": [{
"SucceededCount_MEAS_VALUE": 110
}, {
"SucceededCount_MEAS_VALUE": 120
}, {
"SucceededCount_MEAS_VALUE": 130
}, {
"SucceededCount_MEAS_VALUE": 140
}]
};
list.AllCount = [];
for (var i = 0; i < list.FailedCount.length; i++){
list.AllCount[i] = {};
list.AllCount[i].AllCount_MEAS_VALUE = parseInt(list.FailedCount[i].FailedCount_MEAS_VALUE) + parseInt(list.SucceededCount[i].SucceededCount_MEAS_VALUE);
}
console.log(list);