Can I add a workout afterwards?

Yes you can add it manually in the health app.

In the Health App tap on the browse tab at the

bottom then tap activity and scroll down to workouts

then tap on workouts and in the the top right hand

corner of the screen it says add data,tap on that and

you can enter a new workout and when you're 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.