Hi,
We can create an account directly from the cart but we have to set it before the checkout. That's a nice feature, but most of our customer request it after the checkout/order is complete.
Dynamicweb should be able to get password and username data in any cart step and update the cart/order with the new user ID.
Right now we doing it after the checkout step (receipt) using the API.
Best Regards,
Nuno Aguiar