Render the stuff you want in rpgm from all sides (front, back, left, right) slightly from above (don't know the correct angle, but it shouldn't be hard), make sure to not use a background and no ground enabled, copy the finished render over into existing sprites, watch the sizes and where the original sprite ends (there are templates for sprites, I'm sure you can find some in one of the rpgm forums), delete the original sprite layer afterwards and save under a new name, if something doesn't fit, you'll easily see it in game.