Can you play a wild card on an existing Canasta to go out?
Yes.

You can add to existing melds in general; whether going out or not. This includes adding a wild to an existing Canasta. 


  A player may add additional cards to a meld by his side, provided that the melds remain valid (having no more than three wild cards). 


Note that if you add a wild to a natural Canasta, it will become 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).