Is it possible to add a co-author in conference paper?
The likely answer is yes, but it is the conference committee that makes the rules here. You need to seek advice from them. 

Alternatively, and maybe a better path, is to just do it and submit the paper with the appropriate authors. If the committee has a problem with it they will tell you, but I doubt that would occur. 

But you say abstract in one place and manuscript in another. I'm assuming that the abstract was just a placeholder for what was to come. Acceptance wasn't based on the authors, but on the content of the abstract. Most conferences want to publish and promote the highest quality work. That would probably outweigh other considerations. But they may want to just publish the abstract, in which case they might judge it differently. 

But they make the rules. We don't.