I've had that all along on 8.1 with the Health Vault App.
Health > Measurements > + > input your values.
With Health App, you could add a smart scale...
OK. I just opened Health, saw I had a new message (dot at upper right of Hamburger) > Hamburger > What's New? (which also showed a new message dot) Guess what? Track your weight. It is manual input, just like Health Vault.
Bob's your Uncle now, izzin't he?