How can I add the elevation to the indoor treadmill walk in Apple Fitness?

You can't because your elevation never changes when you're on a treadmill.

I wonder, if perhaps you mean "incline", which can change. Unfortunately, I don't believe that the watch can measure that.

I don't see anywhere in Health that you can add elevation data to a workout. But, again, there is no elevation gain when you're on a treadmill. If the treadmill is located at 50 feet above sea level when you start the workout, it remains at that elevation throughout the workout (unless there is a cataclysmic geologic event, I suppose).