I don’t think there’s a built-in quick way ): To do this quickly, you’d need to write a script that adds N to all tiles within a selection.
Non-script ways:
If your “shadow” tileset is a separate tileset, you could make a copy of your base map and change it to use the shadow tileset as the base tileset by editing the TMX/JSON file. Then, you could copy+paste the parts you need into your base map.
You could use the Select Same Tile tool to select and replace instances of each given tile without accidentally affecting other tiles. This is so slow that I’d only recommend it if you have a fairly small number of tiles, and not 512 of them xP