Technically, yes. A holographic will or codicil can be handwritten on your copy of your will, and depending on many different circumstances, may be considered valid.
However, it’s not recommended to hand-write codicils and other changes, partially because handwritten versions of those documents are almost never crafted with the advice of a lawyer. Minor differences in language that wouldn’t matter in another document can lead to disaster in legal papers, including your last will and testament.
It’s always best to seek a lawyer’s assistance with your will and any codicils, which will usually mean using a typed and properly witnessed document instead of a handwritten holographic will.