Can I move wildcards around, from set to set, to build a canasta?
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.
If you look at the rules for melding once a card is in a meld it cannot be picked back up with one exception for a meld that was played this turn which comes with penaltiess.
After drawing a card, a player may meld cards if (s)he wants to. Cards are melded in columns of at least three cards; e.g. you can meld three Kings, or four Fives. You cannot meld sequences like Four-Five-Six. Once a card has been melded, it cannot be taken back into the hand (except with the Undo meld option).
Note that the rules show a big advantage from not playing wild cards in your melds when possible.
A mixed canasta is worth 300 points, a natural canasta 500 points, and a wildcard canasta 1,000 points.