You may manage your Pre-Order and Backorder in Order Management with the following workflows:

Using the standard in-house warehouse without a specific amount of pre-order inventory

• The sales channel listing SKU would need to push inventory from Order Management to 'False' so that  Order Management would not process 0 available units to the sales channel listing. This means you need to set and manage inventory on channel listing directly. 
• Orders would come into Order Management and stay in an ‘Unresolved - Out of Stock’ status because there was no available inventory in Order Management to allocate for that specific product.
• When inventory became available for the out-of-stock SKU at the warehouse to which the order is assigned, the order would move from an out-of-stock status to a status where the order could be fulfilled.

Having a “Pre-Orders/BackOrders” Virtual Warehouse with a specific amount of pre-order inventory

• A 'Pre-Orders' virtual warehouse would be created and inventory numbers would be tracked in this warehouse. These could be fake quantities or be used to track and sell a specific number of pre-ordered units.
• Using orderbots, orders with a specific preorder SKU could be automatically assigned to a ‘Pre-Orders’ virtual warehouse in Order Management.
• These orders would bypass the out-of-stock status and remain in an Awaiting Shipment status at the virtual warehouse.
• Once the pre-order inventory was physically ready and ready to be fulfilled, the On Hand inventory could be added to the warehouse fulfilling
• Orders and order fulfillment sources could be changed to the correct warehouse.