I was looking at the Sabertooth but I read somewhere that you can only remap to existing buttons, not add buttons. I am kinda drunk so no guarantee on that info.
This is correct. Razer products doesn't magically add more buttons into a game. More or less just remap the keyboard to your liking to use via product.
Take my Razer Naga here for example. There is a little switch button on the bottom that changes the 12 thumb buttons from the #s above the letters on a keyboard to the #s on the Num-Pad.
The Sabertooth would be the same way. It would set up and be used like a controller, but I would assume some of them back triggers I see in pics could be re-mapped buttons (well all the buttons could be re-mapped to one's liking)
, mainly to a modifier key giving you the ability to use like CTRL+1 for a spell/ability.