Originally Posted by
MSPeconomist
Also, it's not a DL rule that PNRs cannot be combined. It's standard across airlines, from when the first reservation systems were designed.
Technically it can be done. However its a very manual/lengthy process. Basically its the same process as when you book a reservation through KLM and call DL to apply GUC. A new PNR must be created by hand. There's no benefit to airline to invest in making process easier.