Billing & Shipping Details Pre-Population
Really sorry for the reply delay. We've been working on getting this new support site set up and had a number of unrelated problems too. Please accept our apologies. We're a really small team trying to do it all!
If you have a look at the docs here for Create Customer you'll be able to see how it works. You're able to enter billing and shipping, save down to the user when you save the order and then on following new orders load them into the order as normal.
Hope that answers your question.