Add heart rate data to an activity later on

3875844 said:

Can I somehow add it to the cycling activity?

Only by editing the FIT file with an external tool. However, it would not be a trivial task anyway. You could also export the activity in the TCX file format - that's easier to edit manually, but it would be still a tedious and a lengthy job. And even if you manage to add the HR to the activity, it will have absolutely no impact on the Training Load anyway, because that's being calculated in the real-time by the watch during the activity.

3875844 said:

And why isn't this done automatically? 

Because it does not make much sense. The all-day HR data is being averaged and recorded with the interval of 2 minutes, and it does not include the HRV and respiration rate data, hence not of much use for calculating such metrics as Training Effect, Training Load, VO₂max, etc.

Thank you for your answer! That makes much more sense now. 

Is there than any way to adjust the training effect / load of that Workout? Currently it's zero load and training effect. 

3875844 said:

Is there than any way to adjust the training effect / load of that Workout? Currently it's zero load and training effect. 

Unfortunately, I do not think there is any simple way to do it. Purely theoretically some 3rd party services like Zwift or Rouvy are able to sync their workouts to the Garmin Connect API, have the device reparsing them, and have then so the Training Effect / Load accounted for. So the theoretical possibility of reparsing synced/imported activities does exist, but I did not hear about anyone yet who managed to hack the system for their personal use. It will be much simpler to do the workout again and have it recorded properly.