I don't see why not. However, there can always be the issue of some of these powers NOT being meant for the base game (for the travel deck, for the new black/white cards, etc.). As a purist, I would advise against using these alternate knights in the base game. Otherwise, if it means having more fun, then knock yourself out.
Ok, here you go. The name of the knight, the name of the knight he replaces (in brackets), and the power.http://www.boardgamegeek.com/article/2435687#2435687
Sir Bors (Sir Gawain) - Any time White cards are shared, you may participate in the sharing, even if you aren't present on the Quest or place where the sharing occurs.
Sir Lamorak (Sir Palamedes) - Any time you are about to lose 1 Life point, you may choose to discard 2 White cards instead.
Sir Caradoc (Sir Bedivere) - Once during your turn, you may sacrifice 1 Life point to draw 2 White cards.
Sir Geraint (Sir Percival) - During the Progression of Evil you may discard 2 White cards in lieu of doing an Evil Action.
Sir Gareth (Sir Tristan) - Draw 1 White card every time you succesfully complete a Move during you Heroic Action phase.
Sir Owain (Sir Kay) - When fighting a Siege Engine, roll the 8-sided die before playing your cards.
Sir Gaheris (Sir Galahad) - During your turn, you may, as a free Action, take 1 Merlin card from the discard pile for each 3 White cards you discard from your hand.
So, a few good knight, but most of them - nothing special.