When picking up a frozen discard pile I understand that you have to have a natural pair in your hand, but can I add those cards to a meld in play in front of me to create a canasta?
Yes, once you have picked up the discard pile, the cards are all in your hand like normal; and you can always add to any existing meld by melding more cards from your hand. Whether the discard pile was frozen or not does not have any relevance here.
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). He may not add cards to the opponents' melds.