Current questions and answers regarding the map editor.

There is also an alternative option using scripts: disable the standard properties of the warrior's tomb, place a prison somewhere in a secluded area of the map, and when entering the tomb, use the function
MakeHeroInteractWithObject(HERO_NAME, PRISON_NAME);

I doubt that this can be done with the available tools; a simplified option is to use TalkBox with answer choices such as "convert 20% of warriors," 40%, 60%, 80%, 100% - I do this for similar things in my games.