You could cheat:
string input = "10072017";
string formatted = input.Substring(0, 2) + "/" + input.Substring(2, 2) + "/" + input.Substring(4);
But a better solution would be to convert it to a DateTime:
string input = "10072017";
DateTime dt;
if (!DateTime.TryParseExact(input, "ddMMyyyy", CultureInfo.InvariantCulture, DateTimeStyles.None, out dt))
{
...
return;
}
string formatted = dt.ToString("dd/MM/yyyy");