Of course you can, however, "Codicil" is an amendment to a will. Amending a trust called "Amendment" to the trust, even if it is just amending a trustee's name. It needs to be notarized and some specific language to be sufficient.
Codicils are amendments to wills. They require the same formality as do wills. Revocable trusts can be amended by a trust amendment, prepared with the same formalities as the trust which is being amended.