Using an existing Canasta to pick up the discard pile
Yes, you can add to a Canasta just like you can add to any other meld.

Source:


  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.


Other rules sources aren't as straight-forwardly clear on that point, but they say you can add to a meld on the table, and a Canasta is defined as a meld of 7 or more cards.