I plan to create quite a few levels using the same “external” tile set, as the game develops I would like to add new tiles and update properties of existing ones in the existing tile-set and have it appear for use in all levels.
Currently, editing the “.tsx” file manually is the only way.
Another way is temporarily importing the tileset and then re-exporting it. This will allow you to add tiles and change properties while the tileset is “internal”.
This is of course a bit of a workaround. Eventually I’d like to support editing of external tilesets properly.
Edit: Editing of external tilesets was eventually added in Tiled 1.0.