As of version 2.1.3, a new option has been added at the very bottom of training menu (lightbulb icon) which is equipment skills acquisition.

This menu can be used to give your heroes a new equipment skills from the corresponding skill manual.

The skill manual will be consumed after the hero learn the skills.

This new equipment skills use the same slot and equipped in the same way as the usual passive skills from sidekick.

You can get two kinds of skill manuals shop exchange of biweekly challenge simulator, which is:

Without these equipment skill, heroes in auto battle will always use the first skill.

You can get these skill manuals from the approval process of office feature: