Slitherine

Is there a way to get the hero Adolf Galland if you missed the opportunity early in the game ? A cheat code, maybe ?

No, any unique hero missed is a forever lost, because the cheat code cannot generate hero with multiple status, and the original event is done by script, which means you have to modify game file itself if you want to trigger it in another scenario.

Iirc there was a thread in the steam forums of this game how you have to modify your gamefiles to make this possible.

You can cut/copy the Galland hero from SCW LUA file and paste it to a later AO scenario LUA file.
Such actions take some trouble shooting, so depending on your Tech Expertise it can be easy to impossible.