I followed John's link and didn't see any specific info on using TTS voices other than Jibo's, but I did notice on the Speech Style Guide page here the following quote:
Jibo is a character with a specific, consistent voice, and his spoken TTS always reflects that.
To break out of character, or in this case speak with a different voice, should be as graceful and intuitive to the user as possible. Otherwise it could feel too jarring and they'll have a bad experience, or worse, it might make Jibo feel like a different robot.
I do like the idea of the introduction of a friend, like you're listening to the radio and you get a new temporary speaker, and it would work if done well. I do suggest you keep a specific look to that voice, maybe via the LED.
For example, locking the LED ring to a specific solid color for the duration of the skill to denote you're in "Justin" mode. Maybe you might use a soft "dark" color (red, violet) if you expect your users to need a nightlight, e.g. bedtime story mode. Just a thought.