Indictments can be amended as the DA's office determines that there are more specific acts which can be proven in court. For example if you are charged with stealing from the cash register on 4 occasions and a fifth is uncovered in a review of the videotape, then yes another charge may be added.
Also, additional charges may be added as "lesser included offenses" which are essentially charging you for something you are already charged for but with a lesser penalty. For example, in regards to the death of one person you may be charged with both manslaughter and negligent homicide, but you can only be convicted of one of the two (or neither)