California Code, Code of Civil Procedure - CCP § 2034.280

(a) Within 20 days after the exchange described in Section 2034.260, any party who engaged in the exchange may submit a supplemental expert witness list containing the name and address of any experts who will express an opinion on a subject to be covered by an expert designated by an adverse party to the exchange, if the party supplementing an expert witness list has not previously retained an expert to testify on that subject.

(b) This supplemental list shall be accompanied by an expert witness declaration under subdivision (c) of Section 2034.260 concerning those additional experts, and by all discoverable reports and writings, if any, made by those additional experts.

(c) The party shall also make those experts available immediately for a deposition under Article 3 (commencing with Section 2034.410), which deposition may be taken even though the time limit for discovery under Chapter 8 (commencing with Section 2024.010) has expired.