WP_Model-serialization.php

<?php
Class Product extends WP_Model
{
    ...
    public $virtual = [
        'humanWeight'
    ];

    public $serialize = [
        'humanWeight',
    ];

    public $protected = [
        'weight',
    ];

    public function _getHumanWeight()
    {  
        return $this->weight . 'Kg';
    }
}

$product = Product::find(15);
echo json_encode($product);