Для реверса:
TimeSpan CurrentTime = (TimeSpan) StoryBoard.CurrentTime;
TimeSpan SubtractTime = CurrentTime.Subtract(new TimeSpan(FrameRate * (long)Math.Pow(10,4)));
StoryBoard.Seek(SubtractTime, TimeSeekOrigin.Duration);
Для форварда:
TimeSpan CurrentTime = (TimeSpan) StoryBoard.CurrentTime;
TimeSpan AddTime = CurrentTime.Add(new TimeSpan(FrameRate * (long)Math.Pow(10,4)));
StoryBoard.Seek(AddTime, TimeSeekOrigin.Duration);