First you should save your excel file to CSV format . Then , Right click the CSV file you just created, click "Open With" and select "Notepad." Next , Click "File" at the top of the window, then click "Save As.". After that Click the drop-down menu to the right of "Save As Type" and choose "All Files.", then ,Click inside the "File Name" field and add ".dat" to the end. For example, if your filename is "model," modify it so it reads, "model.dat." And Click the "Save" button
I did the same way you explained here. But unfortunately the output file is not readable for our model. So, if you know another method, please let me know.
Since I have no experience with BILAN model, so I am not able to provide a clear response. However, when I was working with LISEM model, I remember one of our input files was in ".DAT". I think we were using PCRaster to generate it. Try it.
The other easy solution is to browse and edit one of the default files by software itself. There should be some default files in root directory of the BILAN.