I have read on this board that it is theoretically possible. But in reality, you will need the AS number to physically print on the boarding passes in order to be assured of receiving credit. That means first, after the outbound journey ends to ask the airline to change the FF number and second, be sure that the AS number prints on the boarding passes.

Experience of people posting here is that the airline can not or will not take the time to get the new FF number to print on the boarding pass. So if the mileage doesn't credit correctly, and you might need to send the boarding passes to the airline for credit, they won't credit you because the boarding pass has the NW FF number on it.

It is not dependent on where you purchase your ticket. So buying it through a travel agent, through an online agency or directly from the airline would incur the same problem.

Can't hurt to try I guess.