DataWeave - Add additional item after iteration used for list

I'm trying to create the following XML:

<ns10:Payload>
   <ns6:MeterReadings>
      <ns6:MeterReading>
         <ns6:Readings>
            <ns6:reportedDateTime>2024-06-02T23:15:11.09+02:00</ns6:reportedDateTime>
            <ns6:timeStamp>2024-06-02T23:14:39+02:00</ns6:timeStamp>
            <ns6:value>0.234</ns6:value>
            <ns6:ReadingType ref="0.0.0.6.0.1.54.0.0.0.0.0.0.0.128.3.29.0"/>
         </ns6:Readings>
         <ns6:Readings>
            <ns6:reportedDateTime>2024-06-02T23:15:11.09+02:00</ns6:reportedDateTime>
            <ns6:timeStamp>2024-06-02T23:14:39+02:00</ns6:timeStamp>
            <ns6:value>0.233</ns6:value>
            <ns6:ReadingType ref="0.0.0.6.0.1.54.0.0.0.0.0.0.0.64.3.29.0"/>
         </ns6:Readings>
         <ns6:UsagePoint>
            <ns6:mRID>ES00TRAFGISS03268T120F</ns6:mRID>
         </ns6:UsagePoint>
      </ns6:MeterReading>
   </ns6:MeterReadings>
</ns10:Payload>

I've used the ++ operation to add the element UsagePoint at the end of the list but Mule gives me an error due to me mixing an array with an object.

%dw 2.0
output application/xml
ns ns0 http://iec.ch/TC57/2011/MeterReadingsMessage
ns ns01 http://iec.ch/TC57/2011/schema/message
ns ns02 http://iec.ch/TC57/2011/MeterReadings

var meterReadings = payload.MeterReadings
var meterReading = meterReadings.MeterReading
var readings = meterReading.readings[0] as Array<T>

---
ns0#CreatedMeterReadings: {
    ns0#Header: {
        ns01#Verb: "reply",
        ns01#Noun: "MeterReadings"
    },
    ns0#Payload: {
        ns02#MeterReadings: {
            ns02#MeterReading: readings map ((reading) -> {
                ns02#Readings: {
                    ns02#reportedDateTime: reading.reportedDateTime,
                    ns02#timeStamp: reading.timeStamp,
                    ns02#value: reading.value,
                    ns02#ReadingType: reading.ReadingType
                } 
            })
            reduce ($$ ++ $) 
            ++ ns02#UsagePoint: {
                ns02#mRID: "example"
            }
        }
    }
}

Is it possible to do what I'm trying?

I could not reproduce the issue, probably because you haven't share enough for a full reproduction. However looking at the desired output, you can use the reduce() function to convert the inner array to an object. That will achieve the desired output.

%dw 2.0
output application/xml
ns ns0 http://something
ns ns02 http://somethingelse
---
ns0#CreatedMeterReadings: {
    ns0#Payload: {
        ns02#MeterReadings: {
            ns02#MeterReading: payload map ((reading) -> {
                    ns02#Readings: {
                        ns02#reportedDateTime: reading.reportedDateTime,
                        ns02#timeStamp: reading.timeStamp,
                        ns02#value: reading.value,
                        ns02#ReadingType: reading.ReadingType
                    } 
                }) 
                    reduce ((item, accumulator={}) -> accumulator ++ item)  
                    ++ ns02#UsagePoint: {
                        ns02#mRID: "example"
                    }
        }
    }
}

I used this dummy data as input an changed the expression for MeterReading accordingly:

[
    {
        "reportedDateTime": 123,
        "timeStamp": 1,
        "value": 111,
        "ReadingType": "x"
    },    
    {
        "reportedDateTime": 321,
        "timeStamp": 2,
        "value": 222,
        "ReadingType": "y"
    }
]

Output:

<?xml version='1.0' encoding='UTF-8'?>
<ns0:CreatedMeterReadings xmlns:ns0="http://something">
  <ns0:Payload>
    <ns02:MeterReadings xmlns:ns02="http://somethingelse">
      <ns02:MeterReading>
        <ns02:Readings>
          <ns02:reportedDateTime>123</ns02:reportedDateTime>
          <ns02:timeStamp>1</ns02:timeStamp>
          <ns02:value>111</ns02:value>
          <ns02:ReadingType>x</ns02:ReadingType>
        </ns02:Readings>
        <ns02:Readings>
          <ns02:reportedDateTime>321</ns02:reportedDateTime>
          <ns02:timeStamp>2</ns02:timeStamp>
          <ns02:value>222</ns02:value>
          <ns02:ReadingType>y</ns02:ReadingType>
        </ns02:Readings>
        <ns02:UsagePoint>
          <ns02:mRID>example</ns02:mRID>
        </ns02:UsagePoint>
      </ns02:MeterReading>
    </ns02:MeterReadings>
  </ns0:Payload>
</ns0:CreatedMeterReadings>