I forgot my watch at home when I went for a walk, am I able to add “missed workouts” to a previous day?

To manually add a workout (and its data):

1. In the Health App on the iPhone, tap on the Health Data tab at the bottom
2. Then tap Activity and scroll down to Workouts and tap on it
3. In the the top right hand corner of the screen it says add data (or simply +)
4. Tap on that and you can enter a new workout and its data
5. When you are finished tap on add at the top right of the screen.

Be aware though that this only effects your 3 rings on the day in question. And I heard this MAY not work with watchOS 8.

Yes, you can using the Health app on your iPhone. Open it, tap Browse at the bottom of the screen, tap Activity, tap Workout and then tap Add Data in the top right of the screen.