Where can I change the secondary skills of heroes?

In Heroes 5.5, this skill is actually increased as you want. Ask there if your English allows it :)

My English is limited to the point where I can understand some values in xdb files =)
I can only communicate in it using a translator xD

Yes, I looked into the 5.5 topic, but unfortunately, the result is about the same as I already knew from my own experiments.
There are some skills that have stats in DefaultStats. Everything else is in one well-known place (irony). It is unknown how to access them.