No, it is not legal a trustee may not unilaterally give his or her power to anyone not named as a trustee or back-up trustee, and the appointment of someone not named as trustee in the trust instrument/document will not be valid. Only exception if the trust instruments state that a trustee can do this. The beneficiaries may bring a legal action in chancery court to have the court confirm the invalidity, remove the new trustee, appoint a replacement, and, if any of the trustees improperly took money or property from the trust, to seek compensation from that person.