OK- so I understand when the pile if NOT frozen you can take the discard pile if you can use the top to make a meld from your hand OR take the top card and add it to a meld on the table. The question is- can you take the top card and also add it to a canasta (meld) that you previously made that also is the table?
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).