You can walk around all you want at the international peace park where the arch is located, including "crossing" the border, without having to report for inspection as long as you don't go beyond the boundary of the peace park.
Are you talking about walking to the inspection station and entering the USA or Canada on foot after clearing immigration/customs? I suppose that can be done but then what are you doing for transportation? This port-of-entry is on what essentially is a freeway (although everything is slowed down through the peace park).