Well, here goes.
First, make a new image containing only the tiles you want to replace. Then open up that image in Character Maker Pro, and press SHIFT+S to save the palette as a .act file. Then, open up the tiles you want to insert in CMP (make sure that the image contains ONLY the new tiles), and press SHIFT+L and navigate to where you saved the .act file just now. Select that, and your new tiles will have the same color as the tiles you're going to replace. Then, open up Paint (or any other graphics editor), and open up both the tileset you're going to insert, and the new tiles. Paste the new tiles over the tiles you want to replace, and save the tileset, which is now ready to be inserted.