So Picavoxel does have really basic copy and pasting functionality, but it's quite fiddly to use. If you're looking to make a large-ish single-volume static map at edit time, you will probably find it much easier to use an external editor such as MagicaVoxel. The very latest release of that just happens to have improvements that do pretty much what you're after - constructing large worlds out of smaller parts.
If you're interested in doing this at runtime, that's possible as well by copying voxel data from one volume to another. I actually do this in the Shmup demo by copying from a "tileset" volume to the main playfield as it scrolls along. I can go into more detail on that if needed.
Another option might be to use a Unity tiling asset such as Rotorz. I don't personally have any experience with it, but I understand folks have used it successfully with PicaVoxel volume prefabs as tiles.
Last edited 3 weeks ago