Hello Niklas,
I have recently seen a similar scenario implemented - here also the account was created manually in ERP. The user creating the account in ERP adds the prospect id in a custom field in ERP . This id they send back to C4C during customer replication to map the account created in ERP to the prospect in C4C which effectively converts the prospects in C4C to accounts and updates the external id.
Best Regards,
Pragya