If your question is: Can a creature power work like the hermetic spell Passing the reins of (Spell) but specifically allowing the creature to transfer control of one of its other powers onto a second being.
Then the answer is firstly YSMV but to my mind the reasonable answer is yes.
Creature powers are generally designed like hermetic spells but they have much greater freedom to have powers that work in non-standard ways. Especially if you are designing the creature's powers as a Storyguide.
The real questions are: Why does the creature(or spirit) have this power? and how does this power advance the story(ies) that you are trying to tell?
I would assume that if you let players create a theurgist maga and then design their own spirits then this power is probably highly abuseable. I cannot off the top of my head come up with any specific abuses but being an experienced storyguide my abuse-spider sense is definitely tingling, and I would be extremely surprised if a savy player couldnt use this to pull off some shenanigans.