Third party sales with subcontracting

Dear expert,

I would know if there is a solution in order to cover this process:

1. Customer X place a Purchase Order in Company A (in Germany, 'my' company)
2. Sales Order is created by Company A to Customer X (for third party business, because the goods will be produced in Spain)
3. After the production in Spain the goods has to be delivered from the producer S (Spain) to a subcontracter G (German) for additional manufactoring step.
4. After this additional manufactoring step, the goods have to be delivered from G to the final customer X.

It seems the standard Third Party process but the difference is that I have an additional step and an additional actor (point 3), let's call it "Fourth Party" process.

Please, let me know how could I put in place this process in SAP, my customer is already using Third party process (SO item cat. TAS) and subcontracting (PO item cat. L), but never combined and I think I have to merge both processes.

Many thanks in advance for your clue, I appreciate it....it is quite urgent!

LP