Going Out Concealed in Classic Canasta
Yes, you can do this. There are no special rules or restrictions regarding going out concealed, so you are just subject to the rules for melding normally, along with the rules for going out normally.
Melds belong to both your partner and melds by either of you can be placed in front of either of you; there is only one set of melds for your partnership.

All the melds of a partnership are placed in front of either partner.


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


A player goes out when they get rid of the last card in their hand by discarding or melding it, provided that their side has melded at least one canasta or they complete a canasta while going out. Failing this requirement, a player must keep at least one card in their hand.


A player goes out &quot;concealed&quot; when they meld their entire hand in one turn, including at least one canasta, without having made an earlier meld and without previously having added any card to melds that their partner has made

So it only matters if your partnership has a Canasta, not if you personally do. And you can always add cards to your partnership's melds; it makes no difference if the cards were melded previously by you or your partner. Whether you are just melding some cards or going out makes no difference there.
Source: https://bicyclecards.com/how-to-play/canasta/