First a quick note on terminology: the Reichskommissariat Belgien-Nordfrankreich was a civilian administration which only lasted a few weeks (mid-July to September 1944) before the Allies liberated all of its territory, after which it only existed on paper. For most of the war (from the invasion of France until after D-Day) this area was under the Military Administration of Belgium and Northern France (Militärverwaltung in Belgien und Nordfrankreich), rather than civilian control.

This distinction is important because it explains what you're asking about. The primary reason for the incorporation of part of northern France into this administrative unit was strategic, rather than political or cultural. The Military Administration of Belgium and Northern France was also responsible for maintaining the so-called "forbidden zone" (zone interdite) along the northern coast of France, which included the set of coastal fortifications known as the Atlantic Wall. The two departments of northern France (Pas-de-Calais and Nord) which were included in the military administration weren't technically part of the zone interdite, but they were considered strategically vital to maintaining the coastal defenses of northern France and were therefore left to the military administration until after D-Day. This area was also the site of German V1 and V2 rocket-bomb silos, from which bombing attacks were launched on Britain. This region would've been of vital strategic importance in any future offensive plans against Britain, although those plans never came to pass.

The original plan was to turn Belgium and Northern France over to a civilian administration much earlier than July 1944, but these plans were repeatedly put on hold and eventually shelved until after D-Day. Hitler had originally intended to incorporate Belgium directly into the Reich, which would have created two new Reichsgaue in Flanders and Wallonia. There was also a civilian police presence (Sipo/SD/Gestapo/Kripo) in Belgium and northern France, which was nominally subordinate to the military administration, but in fact operated with a significant degree of autonomy due to pressure from Himmler and Heydrich. There was a separate administrative network for the police forces, with the headquarters in Brussels and regional offices located in most of the major cities in Belgium and Northern France, along with an extensive system of concentration camps, prisons, and other detention sites. The Nazis had the support of local nationalist groups (the Flemish National Union in Flanders and the Rexists in Wallonia), who assisted the Germans both militarily and in the perpetration of the Holocaust and the suppression of resistance forces in Belgium.

It seems as though Nord and Pas-de-Calais were ultimately envisioned as a part of the "Greater Germanic Reich", again primarily for military reasons rather than cultural ones, since that area wasn't populated by Germanic people. However, there doesn't seem to have been a clear administrative plan for those regions, which wouldn't have been part of the aforementioned Reichsgaue. Regardless of the long-term plans, however, the reason for grouping those two departments with Belgium during the war was primarily a military consideration rather than a political/cultural/historical one.

Sources:

Patrice Arnaud and Fabien Théofilakis, eds., Gestapo et polices allemandes: France, Europe de l'Ouest, 1939-1945 (CNRS, 2017)

Wolfgang Benz and Barbara Distel, eds., Terror im Westen. Nationalsozialistische Lager in den Niederlanden, Belgien und Luxemburg 1940-1945 (Metropol, 2004)

Grégory Célerse, Histoire de la Gestapo: Bruxelles, Lille, Paris, Saint-Quentin (Les Lumières de Lille Éditions, 2013)

Dan Michman, ed., Belgium and the Holocaust: Jews, Belgians, Germans (Yad Vashem, 1998)

Werner Warmbrunn, The German Occupation of Belgium, 1940-1944 (Peter Lang, 1993)

Wolfram Weber, Die innere Sicherheit im besetzten Belgien und Nordfrankreich 1940-44. Ein Beitrag zur Geschichte der Besatzungsverwaltungen (Droste, 1978)