The trustor (also called a grantor) is the creator of the trust. Once the trust is created, you can't really add another creator, because that work is already done.But, the current trustor can revoke the current trust, and you can then create a new trust with co-trustors. This may be your best solution. Or, the current trustor can amend the trust to make the new person a trustee (that is, a manager) or to grant the person other powers, such as the right to change the beneficiaries of the trust.  This can be a complicated problem, so you should get advice from a local trusts and estates lawyer.