I have a similar issue to the OP - myself and 2 friends have a flight from INV - SFO coming up in a few weeks. Only myself has status (Platinum) with KLM and all our tickets are the cheapest ones which means only I get a free suitcase included. Our flights include a 2 Delta ones from AMS - ATL - SFO so currently the buy extra luggage option is not available to us on the app as it says we are flying with a different airline (Delta).

Was wondering if one of us upgrades to business on first leg at OLCI would the friendly rep at INV allow an extra suitcase to be added as upgrading to Business might be cheaper than paying on the day at the airport (if at all possible).