Sorry, I made one error. Try this:
public delegate IEnumerable CreateShipmentDelegate(PXAdapter adapter,
[PXDate] DateTime? shipDate,
[PXInt] int? siteID,
[SOOperation.List] string operation);
[PXOverride]
public virtual IEnumerable CreateShipment(PXAdapter adapter,
[PXDate] DateTime? shipDate,
[PXInt] int? siteID,
[SOOperation.List] string operation,
CreateShipmentDelegate baseMethod)
{
List<SOOrder> list = adapter.Get<SOOrder>().ToList();
string msg = "Fix qty for ";
throw new PXException(msg);
return list; // Or: return baseMethod(adapter, shipDate, siteID, operation);
}