Can I move a wildcard after melding if I get 3 natural cards in canasta?
No.

There is no specific rule that I can point to in the various online resources I know to quote; but there is nothing in the rules that says that this would be allowed.

You can add additional cards from your hand to an existing meld; if the rules intended for you to be able to move or remove cards from a meld they would list that as an option.