Is it (or will it be) possible to import data already collected by other apps into the new health app in iOS 8?
As xVir said in the comment, it will depend on the apps and not on Health. After Health has been released, several of the apps I used were updated to support Health.
Some of them just said that they would push data in Health from now on (like Sleep Time), and others took the data they already had created to push it to Health (like FitStar).
So, it's up to the app developer to decide if he implements a way to push to Health the old data the app already created, or only the newly created data.
You can see Health as a "passive" data repository of health information which will be provided by 3rd party apps (or manually). It will then "just" draw charts out of it.
So follow xVir advice and ask the providers of apps. I did it with Sleep Time for example, where I would like to get several months of data in Health (
but no answer for now... Just answered that they are looking into it for a future update)
We recently released a new app that is dedicated to importing data from CSV (comma separated value) data files which covers many formats of exported data from existing apps. It will allow you to import data from apps that have an export feature, but are not yet integrated into HealthKit / Health App.
If this is a feature you are still curious about, please check it out on the App Store at: https://itunes.apple.com/us/app/health-importer/id930943780?ls=1&mt=8