Third Party Registration Form - GSTIN # BY STATE

We use India localization functionality for Payables. We do NOT use Purchasing. We would like to register Supplier GSTIN Number by state (For each supplier site we use). We added following lookup as suggest by Oracle so GSTIN Number can be recorded.

Third Party Registration form provides Primary and Secondary Registration Numbers. We are using Primary  as PAN# and Secondary Registration as SERVICE TAX # for each Supplier with NULL site. When we enter site then same is copied for each site. We can't update at site level.

How do we achieve this? Is our understanding correct to use Third Party Registration Form to enter GSTIN Number?