In one of my three playthroughs of the original campaign, I managed to get Agrael with the Call of Urgash. I didn't use any scanners or codes (I never use either).

I think it will be easier in the latest add-on. Now, for a Specialist Ability, you only need to level up three skills instead of four. Also, if you're lucky enough to find a mentor on the map, you can retrain the hero. However, I didn't manage to get a Specialist Ability in the prologue.

You can view the skill wheel directly from the skill selection menu. In the upper right corner, you need to select the icon of the desired faction.