Although he's completely right, I believe I understand what Scale9 sprites are supposed to do, so...
This sort of stretching logic is at a much higher level than SFML's scope, so it wouldn't be appropriate to put anything like it in SFML itself. However, it should be fairly simple to write a Scale9Sprite class that contains a texture, the margin sizes and the drawing size, and internally uses a vertex array to draw the appropriate subregions of the texture at the appropriate subregions of where it's being drawn. This would be a good project to put on the wiki.
