I don't see why not. From page 54:
Disciples: Phoenix Lords typically lead their brethren into battle. If the Phoenix Lord is leading a squad of their Aspect, then the whole squad becomes Fearless. Phoenix Lords cannot join Aspect Warrior squads that are not of their Aspect. Note that if a squad has two of the same Exarch power, they are not cumulative.
Nothing says that a Phoenix Lord can't join a non-Aspect unit such as guardians or wraithguard. And the exarch powers that a Phoenix Lord has are separate from the Disciples rule, so they would still apply by their terms, and they specify squads, not "Dire Avengers" or "Fire Dragons," etc.
The only exarch power I can think of that wouldn't necessarily apply to a non-Aspect squad is Bladestorm. Bladestorm specifically mentions shuriken weapons, so Asurmen couldn't give it to wraithguard. I'd be kind of irritated if somebody used Baharroth to give Skyleap to a non-Aspect unit, but I don't see anything prohibiting it.