Vasik this was our first time adding new item in the game, guide is very chaotic and probably incomplete. It will be improved. 
Finding assembly reference for ToE is very easy once you have for QAI so that can be done too.
Psatka and Ment are correct - even if done this will only increase the amount of skill slots and nothing more. At best you can use the ID to queue the hero with LUA scripts and give functionality based on that. We managed to add a new skill and assign it to a unit, but it did nothing.

However it is not true we cannot add new functionality, it is possible to add new source directly as assembly reference. For example all these patches add new code at the end of the EXE (and require EXE to be stripped of debug data before that to free space). Only time will tell how much we will be able to achieve.

By the way - https://heroes5.fandom.com is for gathering modding knowledge of all kinds so if you have guides that are not there feel free to write an article.

I spend some time occasionally editing and rearranging what is already there to make it into a more welcoming page, so things will get better over time.