The PowerStore is designed for a single admin user. If you wanted to update it to have more than one admin user you would need to user SecurityAssist to generate a new user authentication login server behavior. You would also need to modify the access rules for administrator to work with the values from your new authenticate user server behavior.
As far as making edits to the PowerStore pretty much all of it can be edited, however any changes that you make to the code or functionality can affect how the site performs. If you want to make edits to the cart object you would use eCart for that. On the support page for Power Store is some guides and info on making certain changes.