Order management extension to only execute for manually created sales orders

Summary:

I would like to create an Order Management Extension to execute on save to prevent users from manually creating a specific sales order. I do not want this execution to run for imported sales orders.

Content (please ensure you mask any confidential information):

99% of sales orders are imported into oracle and each sales order imported is split based on the type of sales order.

For example Drop Ship sales orders are created separately from back to back and fulfilled and back to back are created separately from drop ship and fulfilled etc etc.

At the moment we have no way of stopping the user creating a manual sales order that contains drop ship, back to back and fulfil lines. I would like to create an extension that prevents the user from creating sales orders with mixed flow types but would only want this to run for manually created sales orders. So if the sales orders are imported the extension is ignored.