JSFiddle - React, Tailwind, and code Playground

JavaScript

var json = {
    "cmd": [
        {
            "sales": [
                {
                    "qte1": "2",
                    "qte2": "3"
                },
                {
                    "qte1": "66"
                },
                {
                    "qte2": "77"
                }
            ]
        }
    ]
} 

var formattedJson = json.cmd.map(function(salesList) {
    return salesList.sales.map(function(sale) {
        sale.somme = Object.keys(sale).reduce(function(total, key) {
            return total += +sale[key]
        }, 0);

        return sale;
    });
});

console.log(formattedJson);