As for the second question - you can't. You see, what's on tileset - you can use it. If you want to use more than what is on the tileset, you have to merge two tilesets (or more) using program like Photoshop.
Then you have to go to the database under tilesets, load the new tileset and carefully select passability, priority, etc.
Then load it into a map or start a new map with this new tileset.
SEARCH the web for tilesets, people are merging them.