If I remember correctly, this is only possible when you're using the Frozen Throne editor.
Now, to the specifics.
Go to the Advanced button up at the top and select Modify Tileset.
This will open up a window from which you can add and remove specific tiles from your tileset.
One note, however. There is a specific number of tiles you can choose. That's indicated by the Total Texture Space Remaining number. Keep this in mind along with the "cost" of each tile when adding them!