RE: D365 for Finance and Operations : enqueue file for recurring data job integration inside AX
In general I would question the requirement of not using an integration platform for integrations. You will probably end up implementing a lot of stuff that someone else has already done and published as an integration platform.
But, by examining the source code of RIS, you will see how to send data to D365FFO recurring integration endpoints. Then you could try to write a solution inside D365FFO that first fetches the file from external system, and then sends it to the D365FFO recurring integration endpoint.