this is not possible. If you send a PO to SUS all follow-on documents to this PO must be created in SUS. This includes the confirmation which will create the SES in ECC. Once this is accepted, the supplier can create the invoice in SUS.
If you create the SES directly in ECC for a SUS PO, there is no way this information will be transferred to SUS, so the supplier will not be able to create the invoice.