If you’re looking for the BAPI for uploading journal entry documents to SAP, try: BAPI_ACC_DOCUMENT_POST
This BAPI should allow you to post your documents, but like everything, it will take some work to get it running. We used a custom T-code based on this BAPI for a while, and it was not perfect. One problem we had was that it didn’t validate material numbers when we uploaded them in the entries.
You may save yourself a lot of time and pain by going with a 3rd party tool such as ZOption or WinShuttle. Hope this saves you a couple of aspirin!