I just been successful with adding PTC Login to other accounts that I believe was created using Google ID.
But there are weird processes to be able to do it that I just stumbled upon through Trial and Error…
1st, to be able to make the prompt appear under Account inside Settings, you need to come from another account that uses other form of Login like for me, I need first to Sign in using one of my account with a Facebook Login, then after logging in, you log it out then open your target account using for example Google Login, on my experience, ONLY then that the Link to PTC will be available. weird indeed!
Then, you can now try to Link the PTC account to your existing Pokemon GO Account, you will be required to Login using the PTC Details and then most probably, your effort will be greeted with Login Failure, weirdier isn’t it?
But do not surrender, just try again for another or two and then It could be successful, just like on my own experience
Good Luck and tell us your experience too!
And by the way, why would you go to this inconvenience just to be able to Link a PTC Account to your existing Pokemon Account? Well, as I’ve had read, once linked, it could not be Unlinked for the time being, perhaps in the future, it depends to Niantic of course.
Well as I see it, the PTC Login would give your account another Level of Security in case your Google or Facebook Logins had been compromised, since other person could not unlink the account to your PTC Account, so as long as you have complete access to your PTC Account, any chance that someone may unlink either your Google or Facebook Logins or Both, you still have some sort of a backdoor to your Pokemon GO Account.