Is it possible to add backpack weight to the hiking activity and count it with the statistics?

The closest is to use Gear.

Is it possible to add weight to the gears? I can´t find it in the app. 

I'm looking for the same thing. I'm going often with my son in the backpack and it's about +20kg on my back and it influences the performance indeed...

Would be useful for sure. Valid for hiking & walking where I carry +20  kg about 15-20 km per day. Training effect id almost zero according to Garmin. Hard to believe.  Would also be usefull for Golf when carryingbthe bag (10 kg)

gozifa said:

Training effect id almost zero according to Garmin. Hard to believe.

The algorithm for Training Effect does not use the carried weight. It primarily uses the heart rate, the heart rate variability, the respiratory rate, and the activity class. And then also diverse background data collected from other activities and from the daily monitoring - like for example the VO₂max, LTHR, max HR, resting HR, max respiratory rate, HRV baseline,...

If the TE remains low at your backpacking hike, then it either means that your HR, and other parameters were not significantly elevated, or that the historical background data is inaccurate. Knowing the carried weight is not necessary for the calculation.

Read more details about the way the Training Effect is being calculated, in the white paper from Firstbeat Analytics (the provider of the algorithms; Garmin owned):

EPOC_based_training_effect_assesment_REVISED 2.doc (firstbeat.com)

Just one point to clarify, no Fenix watches use activity class.

mcbit said:

Just one point to clarify, no Fenix watches use activity class.

Activity Class is a setting of Garmin Connect (in User Settings). It is not a device setting. The article What Is the Activity Class Measurement in Garmin Connect? | Garmin Customer Support indeed does not list any Fenix among models using the Activity Class, but it does not list other models like for example Instinct or Instinct 2, and the setting is still available for them too. I do not own any Fenix, hence cannot confirm, whether or not the setting is available for those models.

Another question is whether the setting is used, even if available. It is also possible that at least some devices detect the Activity Class themselves, and that's why they do not need the setting. Unfortunately, this is nowhere documented, AFAIK.

As far as I recall, activity class was used by FirstBeat algorithms (used by Garmin and other brands)  as one of the variables to calculate calories, training effect (although Garmin did not yet report TE to the user at that time), etc. Later on AC was calculated automatically and you did not have to enter the value yourself anymore, but Garmin kept the option on the web interface to faciltate use of old devices that needed it.