Huh, that's a neat idea. I haven't personally seen any papers that explore converting traditional 3D art into splats, but it should be possible. At a very basic level: you could render a 3D item on a black background, capture it from a few different angles, and then generate splats using the conventional approach. I'm sure there would be even more optimal ways to do it, too.
Having said that, you can absolutely blend standard opaque geometry with the splat environment. I just mentioned this in another comment, but I tested it, and it worked quite well with opaque geometry: https://youtu.be/8uB4qC2UtCY