Как добавить массив объектов в запись массива odata

В вашем json отсутствует запятая за каждым атрибутом.

Затем вы можете добавить «masterAttributes», например:

oModel[0].masterAttributes = masterAttributes;

проверьте эту скрипку:

     var oModel= [{
                attributes: [],
                creationDate: "2019-05-20T09:14:16.622Z",
                deleted: false,
                eventDate: "20.05.19"  ,          
                message: "Aufgabe xyz",
                object: "Aufgabe"       
        },
    {
                attributes: [],
                creationDate: "2019-05-20T09:14:16.622Z",
                deleted: false,
                eventDate: "21.05.19",        
                message: "Aufgabe",
                object: "Aufgabe"    }
    ];
    
    var masterAttributes = [{
        description: "Ereignisgruppe",
        key: "objectGroup",
        value: "Aufgaben"
    },{
        description: "Ereignis",
        key: "object",
        value: "Aufgabe",
   }];
    
   // add the other array
   oModel[0].masterAttributes = masterAttributes;
   
   console.info("oModel after adding masterAttributes ");
   console.info(oModel);

    // es6
let oModel= [{
                        attributes: [],
                        creationDate: "2019-05-20T09:14:16.622Z",
                        deleted: false,
                        eventDate: "20.05.19"  ,          
                        message: "Aufgabe xyz",
                        object: "Aufgabe"       
                },
            {
                        attributes: [],
                        creationDate: "2019-05-20T09:14:16.622Z",
                        deleted: false,
                        eventDate: "21.05.19",        
                        message: "Aufgabe",
                        object: "Aufgabe"    }
            ];

            let masterAttributes = [{
                description: "Ereignisgruppe",
                key: "objectGroup",
                value: "Aufgaben"
            },{
                description: "Ereignis",
                key: "object",
                value: "Aufgabe",
           }];
           let newData = { ...oModel, [0]: { ...oModel[0], masterAttributes: masterAttributes}};
           console.info(newData);

Вам просто нужно создать новое свойство и присвоить соответствующие значения

var oData= [{
    attributes: [],
    creationDate: "2019-05-20T09:14:16.622Z",
    deleted: false,
    eventDate: "20.05.19"  ,          
    message: "Aufgabe xyz",
    object: "Aufgabe"       
}, {
    attributes: [],
    creationDate: "2019-05-20T09:14:16.622Z",
    deleted: false,
    eventDate: "21.05.19",        
    message: "Aufgabe",
    object: "Aufgabe"    
   }    
];
var oMergeData = [{
    description: "Ereignisgruppe",
    key: "objectGroup",
    value: "Aufgaben"
   },{
    description: "Ereignis",
    key: "object",
    value: "Aufgabe",
   }
];
//Loop main object and add new property with corresponding values
for( var i in oData) { 
  var oNewData = oData[i];
  oNewData['masterAttributes '] = oMergeData[i];
}