Sign into existing account on STARZPlay Channel for Apple TV

Not really as they are completely separate subscriptions. You would choose one subscription based on your needs. The existing subscription is direct with Starz and uses their app, where the other uses the TV app only and is hosted by Apple (uses their servers, comes with family sharing and offline viewing as well).

Ok??

Its very simple, if you want to use Apple’s channel service then you switch over to their subscription. If you want to be able to use the Starz app then you keep your existing account. You wouldn’t need to have both, and they aren’t the same service so you can’t access the channel when you have an existing Starz account and vice versa. There are other services that work similarly, such as the channels that Amazon hosts within their prime service. Those are the facts, however you are free to do what you want. I will leave you to it.

VicViper9 wrote:

Is something this simple not possible?

That is correct, you cannot use an existing subscription to use the Apple TV channel version within the Tv app as they are considered separate.