Phone the Elite services number for NW- advise them it is a KL reservation (if the record locator is KL generated) and ask for preferred seat selection. They can do it if:
-Your FARE is not in V?? (I think V)
-You are not on a KLM cityhopper flight
-The flight is not within 4 hours of departure
-The flight is not "locked" by KLM (typically due to above)
-It is equal/less than 90 days out from flight time
I just blocked seats for six more flights- no problems except for cityhopper segments (out of luck on those).