I mean a PIN that's not stored in the vault or auto-filled. It would be something extra that you add manually after the password manager fills in the password
So the password manager would put in 'password' and I'd manually type '1234' to make it 'password1234'.
That would not have stopped the vulnerability 'LastPass bug leaks credentials from previous site' (see Zdnet article posted elsewhere) though that's not a common vulnerability in software.
So the password manager would put in 'password' and I'd manually type '1234' to make it 'password1234'.