Legal Question in Civil Litigation in California

No, you must give a name so that the other side can prepare for that particular witness or take their deposition.

Persons most knowledgeable are percipient witnesses, not expert witnesses.

Just to elaborate on the previously given and quite correct answers, the reason is that the expert disclosures are so close to trial that you darned well had better have identified any expert who is going to give an opinion, and have gotten at least preliminary opinions out of them, by the time you must disclose. Otherwise either the opinion should not be important to your case, or you have been litigating a case without any idea if it has any merit.