Can we upload data on SAP customer demand plan?

Dear experts,
I have a problem when create customer demand. I need to create customer demand plans for many products, and I want to upload them by excel, not manually input. Can I input in the file download in Demand Plans called "Demand Plans Key Figures"? Because:
1. Do I need to fill in all columns in this file? Because I just want to upload the name of the products, and use the data already of those products on SAP system.
1. If I input the data in the excel file and upload on Demand Plans. Will it overwrite the data already on the system?
3. Is there any internal data transfer on SAP, itself calculating the most ordered product and transfer to Demand Plan? Or we really need to download and some parts must be done on Excel? Download from this function and upload on another function.