Is it possible to update a running order?

Yes, I have missed an important item on my order, and I would like to include it on my current order.

I don’t know if this is possible as long as the order status is “NEW ORDER”…

What would it be the way to add this missing item??

Try emailing them. they might be able to add the item for you. They won’t be as highly automated as companies like Farnell and Digi-Key; they process orders immediately, dispatching them within an hour or two.

yup, emailing customer service or calling will be the fastest way.