CCP § 2034.310

A party may call as a witness at trial an expert not previously designated by that party if either of the following conditions is satisfied:

(a) That expert has been designated by another party and has thereafter been deposed under Article 3 (commencing with Section 2034.410).

(b) That expert is called as a witness to impeach the testimony of an expert witness offered by any other party at the trial. This impeachment may include testimony to the falsity or nonexistence of any fact used as the foundation for any opinion by any other party’s expert witness, but may not include testimony that contradicts the opinion.