better question. Yes you can (replace only), you will need to import the cliff tiles you want to use into the game, even if they are standard warcraft cliffs from a different tileset. However, you can have only 2 types of cliff tile per map (1 rough and 1 straight) so if your idea includes more then 1 cliff type in the map it won't work. If all you are looking to do is add a cliff tile from a different tileset into your map then check out this tutorial, it will teach you what you need to know.
One thing to remember is that (I'm pretty sure) the reason you can't swap cliffs into different tilesets is because they use a specific tile around the base of the cliff... which you will also need to re-import if you want your cliff to look proper. You will need to import this extra tile and set it's path to the path of the tile that is normally around the base of that specific cliff.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.