For this sample code I used a Regular Expression. The same thing can be done, converting the number to a decimal type and using a ToString("### ### ### #####") call. I used an extension method so you can call it this way: "12312312312345".ToSiretNumber() and the result will be "123 123 123 12345".

    public static class StringExtensions
    {
        public static string ToSiretNumber(this string helper)
        {
            string siret;
            try
            {
                siret = Regex.Replace(helper, "(?<first>([0-9]{3}))(?<second>([0-9]{3}))(?<third>([0-9]{3}))(?<fourth>([0-9]{5}))", 
                    "${first} ${second} ${third} ${fourth}");
            }
            catch (ArgumentException)
            {
                siret = helper;
            }
            return siret;
        }
    }

If the string is not a valid SIRET number the same string is returned.