MrZub, you place a dungeon, select it, press the spacebar, choose Custom in the window that opens, and then select Agrael. Okay. Done.
I've already tried that. Agrael isn't in that list – it only contains multiplayer heroes. Maybe something needs to be enabled in the advanced editor, but I don't understand which module is responsible for that. I tried clicking on several that I thought might add heroes, but they still didn't appear in the dropdown list.
But thanks for the advice.