Add this to the category of "dumbest default behavior". I'm really thankful to find this discussion of it. I was NOT able to just simply drag and drop my already configured canvas to before my sketch, which is incredibly annoying, but I was able to rewind the timeline and completely recreate my canvas, scale, and position it and then fast forward again to my sketch. So, ultimately I was able to work around the issue, but I find it incredibly dumb that the developers think that people know right up front which canvas they want to use before they've established any kind of reference feature.
Use Case: I have a feature on a picture / canvas that I know is a certain dimension. I can use this simple feature to establish both the scale of the picture/canvas AND the position of the canvas. Well, first I want to draw my known feature.... in my case a 51.2mm sized hole. THEN I want to be able to add the canvas and have it show it in the sketch I'm editing so that I can scale it / position it properly and then USE it. I shouldn't have to know ahead of time that I want to have a canvas, I ought to be able to "force" or "choose" that the canvas be visible when I'm editing ANY sketch regardless of when I created that sketch.
Anyway, again thank you for this discussion as it fixed the problem I was having, but **** if the developers aren't ass-backwards in their thinking on this.