Another question I find myself running in to, I've searched the wiki and can't seem to find anything on this subject.

Let's suppose I want to, say, add plants or other in the outside of whiterun? Like, do I just have to load a cell?

Yup. Load a cell near Whiterun from the Cell Window (Look for WhiterunEntrance or something like that), then find the objects you want in the object window. Drag them into the render window and place them where you want them.