Yes, if it gets accepted by the lockbox, you can try to submit additional evidence directly to the California Service Center in Laguna Niguel, CA. We did this for a missing item.
Make sure you write a very concise cover letter explaining that it was omitted from your petition, and requesting that the enclosed evidence of the USC petitioner’s citizenship be included in the file. Make sure that your letter contains ALL relevant info, starting with the receipt notice number (will be WACxxxxxxxx Something), and full names and DOBs of both petitioner and beneficiary. Good luck!