I have read a few articles about moving data out of SAP BW and into SQL Server. I cant find any articles on moving the data from SQL Server to SAP BW, is it even possible an
I guess the easiest way would be to use an ETL tool to extract data from MSSQL and insert into BW; specifically Microsoft's Integration Services (SSIS), which you likely already have with your SQL Server license.
SSIS is a visual tool specifically designed for ETL scenarios like what you described, and is easy to get started with, I'm guessing that this tutorial covers 90% of what you'll need to do.
However, you should also investigate if your license allows you to use the SSIS connector for SAP BW and test if it is a viable option to simplify writing to BW, as writing directly to BW's tables may not be an option (e.g. because of licensing restrictions and/or technical impact due to directly manipulating a closed system's database).