For my trinket, I wanted to make the front side unmasked copper, since it's shiney and reflective and looks cool. KiCad supports converting graphics to component files, but only for the silkscreen (user text) and soldermask (the green stuff) layers. But, with the new plaintext components, it's trivial to get graphic copper. Here's how.
Start by opening Bitmap2Component. From the main KiCad window, it's the icon to the side here. That'll open the program shown below. You can follow the usual way of creating a component: click on Load Bitmap, adjust the threshold value to select which parts are in the component file, and so on. The Black&White tab shows the final outline. For copper, try to use a resolution above 300dpi (600 or 1200 is best); modern processes can create really, really small features, and you want to take advantage of that. When you're done, save it as a Pcbnew file (.kicad_mod). It doesn't much matter which layer you use, but I'll pretend you're using the Front Silk Screen layer.