It might be possible now back then no. They added this new feature recently where if commanders like you they will reward you troops and items at random and I remember being given exclusive flag bearers for Isengard and citadel guards for Gondor so i'd imagine Mordor commanders could possibly give you them at random but other than that they're exclusive.

Hi, I don't think there is a list, at least I'm not aware of it. There are two ways to get these troops. First one is to get high enough relationship with a commander. In my experience this is around 35. They will start awarding you with either exclusive troops or very good gear when you exit conversation with them. It has a cooldown tho, around a week in my opinin, not really sure tho. The second way only works for some troops and it is rescuing them from imprisonment. Since orcs cannot be prisioners, it doesn't work for them, but I remmber getting quite a few King's guards this way,