I shared a link with friends and then realized others people might be interested by the content. But I don't see any button that allow to update/edit sharing list.
I know I can re-share the posts, but this doesn't keep the existing comments.
So is there a way to add a user to an existing post or is this missing on purpose?
This feature is missing for a reason. Not necessarily a good reason, but certainly a reason.
The idea is that the people who were permitted initially may be responding the way they are with the understanding that the share was limited, and opening it up to more changes that expectation. This is particularly true if they've already replied to the message.
It is easy to poke holes in this argument, however. The easiest point is that you can add more people to a share, although it still may not quite do what you expect.
If you share a post with a circle, and then add more people to that circle, then the new people will be able to see any older posts that were shared with that circle. They won't get notification about the older ones, but will be able to see them in their stream and in your profile.
If you haven't locked the post, you can also add people by +mentioning them as part of a comment. This will send them the notification, but also expose that you invited more people to everyone else participating. (If you already locked the post, you can unlock it long enough to add people, then re-lock it.)
There are also other places in Google+ where you can change the sharing level. You can change the visibility of an album, for example, tho if you open it up too far all old comments on the album are removed. With some limitations, you can invite more people to a hangout.
As with everything in Google+, if you don't like how this is implemented, make sure you use the Send feedback feature to make sure your concerns are heard.
