I subscribe to Norton VPN under a different username and password.
When I attempt to download the Norton Security through Xfinity I am asked to create a new account or existing user....
You could try using a different Email address to create a new Norton Account (used only for your Comcast Norton product).
You would still have to use your current email (user name) and password for Norton VPN.
I am curious as to why it prepopulates a PIN - is it in fact a 13 digit PIN or more like a 25 digit Product Key? I do believe if yu have a purchased Norton VPN it would a product key in lieu of a 13 digit PIN.