How can I import external data into HealthKit?
Third party developers can provide import functionality to health app via the normal Health Kit API on iOS.

Some apps scrape or use web API such as Sync Solver exist to get FitBit data from the cloud to the local health app on iOS. 
Some such as Health Importer are more general. (proper disclosure: I work on this app)

We have an app on the store that is designed specifically for importing arbitrary CSV data into HealthKit / Health.App.  

Here are a few screenshots from the development version to show how the app works:

CSV's can be sent via email and then opened in the app:





You can then configure each of the columns within your CSV to a supported data type from HealthKit / Health.App:



After that, just tap the import button and it'll bring the data in: