Het toevoegen van extra properties aan een class object

class Customer implements JsonSerializable {
 
    public $firstName;
    public $middleName;
    public $lastName;
    public $email;
    public $phoneMobile;
    public $gender;
    public $zipcode;
    public $city;
    public $address;
    public $houseNumber;
    public $houseNumberExtension;
    public $birthDate;
    public $iban;
    public $setAVG;
 
    public function __construct($data){
        $this->firstName = $data['fname'];
        $this->middleName = $data['prefix'];
        $this->lastName = $data['lname'];
        $this->email = $data['email'];
        $this->phoneMobile = $data['phone'];
        $this->gender = $data['gender'];
        $this->zipcode = $data['zipcode'];
        $this->city = $data['place'];
        $this->address = $data['street'];
        $this->houseNumber = $data['houseNumber'];
        $this->houseNumberExtension = $data['extension'];
        $this->birthDate = $data['birthdate'];
        $this->iban = strtoupper(str_replace(" ", "", $data['iban']));
        $this->setAVG = $data['avg'];
    }
 
    public function jsonSerialize() {
        return [
            "FirstName"=> $this->firstName,
            "MiddleName"=> $this->middleName,
            "LastName"=> $this->lastName,
            "Email"=> $this->email,
            "PhoneMobile"=> $this->phoneMobile,
            "Gender"=> $this->gender,
            "Zipcode" => $this->zipcode,
            "City"=> $this->city,
            "Address" => $this->address,
            "HouseNumber" => $this->houseNumber,
            "HouseNumberExtension" => $this->houseNumberExtension,
            "BirthDate"=> $this->birthDate,
            "IBAN"=> $this->iban,
            "SetAVG"=> $this->setAVG,
            "Country"=> 'Nederland',
            "Language"=> 'nl_NL',
            "UseValidation" => true,
            "IsValidated" => true,
        ];
    }
}