I checked the NWA website about redeeming an award. It said that a stopover is allowed on most international trips. Travel must be via a published route and must not exceed the maximum mileage permitted.

What is the maximum mileage permitted?

Here is it.

Open-jaws and Stopovers

When you travel on a WorldPerks award ticket in some travel regions, your travel may include either one single open-jaw or one en route stopover per roundtrip journey, not both.
An open-jaw is when you travel to one city and return from another, or vice versa; for example, flying from Boston to Billings and returning from Denver to Boston. In this example, the open-jaw segment is Billings to Denver. Both flight segments must be of a greater distance than the non-flight (open-jaw) segment. If the open-jaw portion exceeds the maximum permitted mileage, two roundtrip award levels are required.
A stopover is a planned break of more than 24 hours on an international trip or four hours on a domestic trip between your point of origin and your destination or vice-versa. One free en route stopover is permitted on a Northwest, KLM or Continental flight from North America to Asia, Europe or Hawaii. Stopovers are permitted only in a Northwest, KLM or Continental hub, or at an international gateway city that is indicated on your flight itinerary. Free stopovers are not permitted for travel within Asia Pacific/Micronesia, or within/between the 48 contiguous United States, Alaska, Canada, Central America, Mexico, the Caribbean and Puerto Rico. Travel must be via a published route and must not exceed the maximum mileage permitted.