How to default Supplier Type in Register Supplier and Manage Supplier page

Summary:

We have a business requirement to register new supplier with supplier type defaulted to Supplier only. The team responsible for new supplier registration should not be able to select anything other than Supplier. Once the supplier registration request is approved, supplier type can be changed (By another team) to any value other than Supplier as per requirement from Manage Supplier page.
To achieve this, we used sandbox to default Supplier Type value Supplier from Register Supplier menu.
But once the supplier registration request is approved, supplier type becomes blank in the Manage Supplier.
How the Supplier Type value Supplier can be defaulted from Register Supplier page and this value will be pertained in the Manage Supplier page?