Hi Prabha,
Thanks for trying the new Entities feature.
Unfortunately, you can't make Entities out of old Submissions because those Submissions were created using a Form that didn't have entities sheet and save_to details.
Regarding uploading CSV file to EntityList, that feature is in the pipeline. For now, you can use Create Entity API to create entity from the CSV file. If you know python you can use pyodk; If you don't know programming then you can use Postman for consuming APIs. Here is a short tutorial on how to use CSV file in Postman to upload data in bulk.