Can we add an acknowledgment to a paper after it has been published?
What stage is your paper at?

If your paper has been published: no, you cannot. It's too late. The electronic files have already been distributed and it's possible the hardcopy versions have been printed as well.

If your paper has been accepted but not published: you can, but hurry. Tell the editor/publisher you want to add an acknowledgements, and give them the exact text. Chances are they won't object. The earlier you do this the more convenient it'll be for the publisher. If you wait till after you've sent in author corrections, they might say it's too late.