It is possible to edit a post and add tags up to 5 tags. If you had 4 tags, you can edit and add an extra tag. If the post has 5 tags already, you can delete one and add a new one. You can never edit the first tag as it is considered to be the "topic" of the post.

You can always create new tags. You can use any word as a tag in your posts.

Do you mean something else? If so, please clarify what you meant.