Short answer. Not by simply referring to it. A will that is void as to its form cannot be republished by a later will or codicil unless the testator reproduces its dispositions in that new instrument. Pointing back to a formally defective will is not enough; its contents must be written out afresh with proper formalities.
What the law says
The testator cannot republish, without reproducing in a subsequent will, the dispositions contained in a previous one which is void as to its form.
Civil Code, Article 835 — Republishing a Void Will. Read the full provision →
A form-void will cannot be revived by reference
Article 835 draws a sharp line for wills that failed on their formalities: the testator cannot republish, without reproducing in a subsequent will, the dispositions contained in a previous one which is void as to its form. If the earlier will was defective in execution, the wrong number of witnesses, a missing signature, an attestation that did not comply, it never became a valid will at all. A later instrument that merely says "I confirm my earlier will" cannot breathe validity into it. To give those dispositions effect, the testator must set them out again in the new, properly executed instrument.
Why reproduction is required
The reason is that there is nothing valid to bring forward. Republication by reference works by re-adopting a will that was validly made and dating it anew; it presupposes a genuine will underneath. A will void as to form is a nullity, so a bare reference points at nothing the law recognises. Allowing a codicil to validate it by mention would let the testator sidestep the very formalities the earlier will failed, the safeguards that protect against fraud and mistake. Requiring the dispositions to be reproduced forces them through the formal requirements once more, this time correctly.
Contrast with a will that was validly made
The article's restriction is specific to formal invalidity. A will that was properly executed stands on different ground: a later codicil referring to it can republish it, carrying it forward as modified, precisely because there was a valid will to republish. So the pivotal question is why the earlier will is being treated as ineffective. If it was good in form and merely revoked or superseded, reference may revive it; if it was void in form from the outset, only reproduction of its contents in a valid instrument will do.
The practical route
If you are holding a will that failed its formalities and want its plan to take effect, do not try to patch it with a codicil that gestures back to it. Have the dispositions rewritten into a fresh will executed with all the formal requirements observed, or reproduced within a validly executed codicil. That new instrument, not the defective original, is what will be offered for probate. A lawyer can identify exactly where the earlier execution went wrong and ensure the new one does not repeat it.