There are two purchases. The item purchased from Supplier A , and the processing done by Supplier B.
Add the Item , and in Supplier Details check Delivered by Supplier (Drop Ship), so that the Item can be drop shipped to Supplier B.
Add a second Item for the Supplier B service. Uncheck Maintain Stock. It may be advisable to create an Item Group for services. Create a Purchase Order for the service, which can include processing and delivery in the Description.
That’s probably as simple as it can get without BOM and the like. The accounting aspects of it , if a per item allocation of costs must be made , would require a different approach, if a simple COGS won’t suffice.