Unless you're a reseller (have access to reseller API), you'd have to do them individually, as even if they are all under the same account with the exact same details (whois, etc. - ALL other data - being the same except EPP keys), those keys are domain specific (your issue to begin with). If you're a reseller, you'd have to custom-code a back-end script to search for the EPP keys in the data returned via the API.