Have you tried to insert a record to Microsoft Dynamics 365 Business Central through a Custom API and pass a custom SystemId with the record?
Basically what I trying to achieve is when an Account is created in the D365 Sales system, I want to pass that record to D365 Business Central. I can easily do that using D365 Business Central APIs. However one of the challenges I faced was how I going to do the mapping between D365 Sales records and D365 Business Central Records.