Possibility of getting HealthKit data from third-party sensors in real-time?
From a user level - the health app consumes data that is stored in the HealthKit framework. From the developer and hardware manufacturer side, HealthKit framework is an API for them to do things reading and writing the data so that what you describe is clearly possible.

Even watch OS streams live data - so that's a core feature of bluetooth low energy. Once the data is on iOS, it's a matter of the developer placing it under health kit control. This is something Apple expects and allows with their code and SDK/API.

Now, not all data can be written to all areas - for instance, Apple's workout app doesn't allow third party contributions on Watch OS 1.0 and 2.0 but it does allow workout data to be stored in the database along with the Apple watch workout app data.