How do I upload parents without overriding the existing users of Foundry?
It's really very simple:
Uploading doesn't replace, but rather always adds information that wasn't there before.
So you can upload a list of entirely new users using the Foundry provided template and they will be added to the list.
If you need to add or modify information about an existing user, you will download not the template, but a user extract as a spreadsheet. You then make the modifications you need to make and upload that spreadsheet in the same manner as the template.
You can verify upload by going to the 'Users' page, and add the new users to a group in the 'Groups' page, both in the admin role side of Foundry.