Submitting at the mediation conference may not insure that the Court ever will see the same. I am not against your doing so, but if you have proof of these other accounts the best way of obtaining some denials and then an admission of their existence is to have your attorney take his deposition.
If upon being asked at deposition to reveal all accounts, he denies the others you have proof of, your attorney could produce the records after he denies and you will have him under oath being deceitful. This might also prompt him to then reveal others.
Your attorney could also ask the Court to have your husband sign an unrestricted release allowing you to obtain any and all records from any financial institution allowing you to go on a hunt of your own to get the records. You would think that a copy of his credit report would also be required to be produced, which may have more information as to accounts and the like.
An attorney can also subpoena those bank records and perhaps trace where those monies went. This may or may not be cost-effective, depending on how much you believe is there.
Disclaimer: This answer is intended for informational and educational purposes only and should not be considered legal advice or forming the attorney client relationship. Please consult a lawyer before taking any action.
You can subpoena the banks' records. It takes a MINIMUM of 24 days to get bank records, so if you are headed to mediation soon, you may not have those records. However, the issuance of the subpoena might give your spouse notice that you plan to investigate this carefully.
When you get the bank records, you'll introduce them in court as evidence and it will become evident that your spouse lied as to the nature and extent of the marital estate.
If you don't have an attorney, you need to hire one right away. Do not go to mediation alone. You must have an attorney.