Hi,
I don't see the PIN number in the customer master now nor neither I saw ever.
If this is something to add only information as PIN number then I think you can add this number into any unused field and change the name of that field, such as Known as, Registration IDs, etc.
However, if this is something to with based on the PIN number, there will be control to access these customers, I think then you need to customize this process by XDS function, data level security.