As SuziQ says. The DLCs don't add levels to the campaign, but all new modes have a new "level" in the background.
The reason why we opted for Modes instead of expanding the campaign is that the Modes offer way more replayability. I saw several people burning through the campaign levels and considering the game done after beating the final Boss at Rookie, when that is barely "warming up"
Also, adding new levels to the campaign kind of invalidates all the current arcade scores, so it definitely wasn't our best option.