I discovered a cool app today: Health Importer. The app takes a .csv file with data and imports it into Apple's Health app. It recognises dates and you can match the other fields in your csv to any kind of data that Health accepts.
I had an old backup from Pedomater++ sitting in my Mac, and the app smoothly imported all that old data into Health. In theory you could use the same do import existing data like weight, heights,... into the app.