You can create and add products to a Sales Order (SO) regardless of whether the ordered products currently have any stock on hand available. You can save the Sales Order in a Parked, Placed or Backordered status, whilst waiting for the ordered product to become available. 

Whilst you cannot ship the ordered products and complete the Sales Order if there is not enough stock on hand available to fulfil it, if you have the Order Management setting enabled you can still complete an invoice for it and manage the payment in advance. The shipment can be dispatched at a later date, when stock is available, to finally complete the Sales Order.

For more details, please see the below tutorials:

• Create a Sales Order
• Sales Order Managing Invoices
• Sales Order Managing Shipments