Legal Question in Civil Litigation in California

I'll give you the classic attorney answer: It depends.

You haven't provided enough information for us to know what the relationship of the parties is, or whether the defendant has counsel, but it may be possible. The settlement agreement would only be valid as to that defendant if he or she signed his or her name to it. As long as that defendant agreed to the terms, and signed the agreement, then the answer to your question is probably 'yes.' Results will vary depending on the circumstances.

I don't see why not, given the limited information you have provided. I've drafted settlement agreements that include parties that I don't represent, and then send it on to either those parties or their attorneys for review and signature. Is that your question?