Adds multiple fields to a DE using SSJS. Loops through a DE to get the customer key.

SSJS Add Fields.ssjs

// Name AND customer key of DE are: thisIsTheSourceDe
// Field in the above DE is: myCustomerKey (text)

// https://developer.salesforce.com/docs/marketing/marketing-cloud/guide/ssjs_dataExtensionFieldsAdd.html

<script runat="server">
Platform.Load("core", "1");
var api = new Script.Util.WSProxy();

try {
    // Define the customer key of the data extension that contains the 1000 customer keys
    var dataExtensionCustomerKey = "thisIsTheSourceDe";

    // Retrieve all rows from the data extension containing the customer keys
    var dataExtension = DataExtension.Init(dataExtensionCustomerKey);
    var data = dataExtension.Rows.Retrieve();

    // Check if any rows were retrieved
    if (data && data.length > 0) {
        for (var i = 0; i < data.length; i++) {
            var customerKey = data[i].myCustomerKey;

            var fields = [
                {
                    Name: "fieldnameOne",
                    CustomerKey: customerKey,
                    FieldType: "Text",
                    IsRequired: false,
                    MaxLength: 30
                },
                {
                    Name: "FieldNameTwo",
                    CustomerKey: customerKey,
                    FieldType: "Number",
                    IsRequired: false
                }
            ];

            var result = api.updateItem('DataExtension', {
                "CustomerKey": customerKey,
                "Fields": fields
            });

            Write("Updated fields for CustomerKey: " + customerKey + ", Result: " + Stringify(result) + "<br>");
        }
    } else {
        Write("No records found in the data extension.");
    }
} catch (error) {
    Write("An error occurred: " + Stringify(error));
}
</script>