How to add additional navigational properties that won't be persisted?

I have the following model:

public class Order
{
    public int Id { get; set; }
    public ItemType ItemType { get; set; }
    public int ItemId { get; set; }
}

public enum ItemType
{
    Customer, Organization, Department, //etc
}

The ItemId is a foreignkey to a separate model/table. The above model maps as-is with the underlying table.

Now I would like to automatically load the related entities when loading an Order.

Is this possible by using composite foreign keys when mapping, as in:

public class Order
{
    public int Id { get; set; }
    public ItemType ItemType { get; set; }
    public int ItemId { get; set; }
    public ItemTypeCustomer => ItemType.Customer;
    public ItemTypeOrganization => ItemType.Organization;
    public ItemTypeDeparment => ItemType.Department;
    public Customer Customer { get; set; }
    public Organization Organization { get; set; }
    public Department Department { get; set; }
}

modelBuilder.Entity<Order>()
   .HasOptional(p => p.Customer)
   .WithMany()
   .HasForeignKey(p => new { p.ItemTypeCustomer , p.ItemId });

Would the above work? Is it even possible to build something like this using code-first?