Hi Michel - If I understand correctly you want to "push" data into OS via a stream or post method of sorts from SAP or any external system?
I am sure you have already explored, but it's far easier typically for OneStream to call out to SAP via direct connect, SOAP API (BAPI layer), via a Business rule to "pull" the data into OneStream. There's probably a good reason/s reasons for not allowing this type of "push" method in the application just given the nature of the type of data that OneStream typically holds and the focus on data quality and workflow when loading the data.
That being said, there's probably a few options "workarounds" depending on what exactly you're trying to do. One I can think of that might work would be to call the FileExplorer API and "push" a stream of data to a file within the OneStream folder structure. Once this file is there, then the normal batch harvest process would fire off and load the data. I suspect you would have to do some sort of conversion on the encoding of the file contents on streaming, but in theory this could work.
Similarly, if your calling application could push a file to an sftp folder, OneStream could pick up the file from there and load. You then could have a BR/Data Management job that would fire off every so often to check for that file and load it.
Alternatively, I suppose you could host your own API somewhere to broker the interaction between SAP and your OneStream instance; probably a bit more complex and would require custom dev work.
I'll post back here if I think of anything else. Good luck! Will be interesting to see what you come up with here.