I was thinking of a GUI of system drives at the Where to Backup screen

Where are we in the app? In the backup wizard? I need some clarification here.

...making a storage account, then going to users...

If you're talking about onboarding a new customer, the easiest way to do that is:
1 - Create the customer (from the Companies screen) and assign the storage account here
2 - Create the customer's service account (from Users screen) - this can simply be a username / password and assign to customer (no need to assign storage here)
3- Your customer is ready for agents to be installed and assigned using that User account

" I was thinking of a GUI of system drives at the Where to Backup screen"

I'm talking about in the wizard on the website. After thinking about it, perhaps this is unnecessary and I just need to configure a Storage Account for File System (which I have done) and get all the drive letters on external USB drives to be consistent.