They do, prior to sentencing. Every jurisdiction that I'm familiar allows for the entry of a victim impact statement prior to sentencing. Some jurisdictions allow that this statement be read aloud, in court and by the victim themselves, or by their family if the victim is deceased.
However, most jurisdictions, including federal statutory and case law, limits the participation of potential witnesses to include them being in the audience for any portion of the case that does not involve their direct testimony.
I know that's not the way it goes in TV and the movies, but that's done in furtherance of artistic and dramatic license, not factual accuracy.
Thanks, interesting.