Certificates must be added prior to generating a production build. You could add your own certificates, but your customer would not be able to change them to their own once you send the files along. Your customer will need to provide you with the needed certificates.
The same is true for the mobileprovision. The mobileprovision is required to generate all iOS builds.
In Android, could I use VoltSigner to sign with a selfsigned certificate (o even no signing at all), and then my customer can use Play App Signing later?
Debug Android builds don’t require certificates, but production builds do. With Play App Signing, the certificate you sign your app with is the upload key. You will want to provide that to your customer. Once uploaded the app is signed by the play store.