Actually, no. Once segments are flown, CO agents have no history of NW segments. NW can print you a receipt for your flight, and you can send that in for credit.
If the segment is less than 2K, ask the agent to put it on the manual log. (It's the agent's discretion), but if the agent won't do it (could have done too many for the day, so be nice) just ask for a supervisor who should be able to add it for you. Especially if one leg has already posted to your account and the ticket number is in the posted record.