Problem
Build Orders sometimes require sending an item externally for processing (e.g. anodizing) prior to completion .
Ideally, the Build Order module would allow you to mark an item as requiring external processing and then allow you to pull the item to a purchase order or sales order (sometimes called a Repair Order in other systems).
This would require a change in the purchase order/sales order module.
This could potentially tie into issue #2817 as it would be a part of build routing.

Suggested solution
Assuming build routing is implemented as described in issue #2817, each routing step would have the option to have a property indicating whether the step is internal or external.
If the property is labeled as external then it becomes possible to label the items as corresponding to a specific purchase order or sales order.
If using the purchase order module, the purchase order could then have a virtual P/N corresponding to the external processing step and would additionally have meta data notifying the user of the link to the BO.
The purchase order module would then need similar structure to the Sales order module for creating shipping.
Alternatively the BO could be pulled to a sales order even though the build outputs aren't complete if it is marked as an external process item.
The sales order module would then just need a way to differentiate between selling something and sending something out for external processing.
I think integrating it into the sales order module would be preferable to the purchase order module as most of the structure seems to already exists regarding linkages to build orders.

Describe alternatives you've considered
You could potentially use the test framework for this in conjunction with a SO however that would not allow you to pull items from the partial BO to the SO.
A custom report could be used to create a form to send to vendors based on the SO to complete the required work.

Examples of other systems
The suggested solution above is similar to how most ERP systems handle routing of items for external processing although they sometimes refer to Repair Orders instead of Sales Orders or Purchase Orders.

Do you want to develop this?
Yes, I have experience in python. I am not sure if this is feasible as a plugin or would require some other approach.