KM told me it is not posible.
But it is possible:
I booked a round trip flight PRG-AMS-MLA on 22FEB and wnted to credit the miles to my Delta FF account. Those with KLM did credit OK, but those flights with KM Malta did not (of course). But since the ticket was bought with KLM, I could claim missing miles to thr Flying Blue account. I just entered the ticket number and voila! 323 Miles and 5 XP were credited immediately for the one flight. It says it is "too early" for the sreturn flight to be credited. Not that I will use it, but probably better to have it - one never knows if I decide to switch from devaluating Delta to Flying Blue.