Canasta- using the top card of a non frozen pile to a canasta meld
Yes, a Canasta is not diffferent than any normal meld when it comes out adding more cards to it. You can add another card to your Canasta whether that extra card comes from your hand, or if it is the top of the discard pile.

From Bicycle cards: 


  A completed canasta is squared up with a red card on top to indicate a natural one and a black card on top to indicate a mixed canasta. Additional cards may be added to a canasta to score their point values, but these do not affect the bonus - except that a wild card added to a natural canasta reduces it to a mixed canasta (and a black card replaces the red card that was previously on top).