Hello Rik!
We use a different approach (Purchase Order with subcontracting indicator instead of Process Order.) The material has to carry a BOM too and the procurement type is X (both external procurement and in-house production).
Anyway, I don't know your process, but have you already checked if it is necessary a info-record for the subcontracter (as a vendor)? As you might know, you have to add commodity code in inforecord too...
Hope it can shed some light...
Rgds,
Marcio