Seconded. This is an excellent idea and an aspect of Aseprite that turned me a bit off when I first got into it coming from Graphics Gale.
In Graphics Gale, you could simply load an image with the palette you wanted (it would just grab all pixels and ignore any duplicate colors, forming a palette out of the first 256 pixels it found), and that was easier than the palette options Aseprite had. However, this one would be a game-changer if you could in fact load your palette from an image in Aseprite too – and then organize it like the above. D: