Does anyone know how to connect a Roku smart TV to my existing account?

To add to what AvsGunnar said, if that TV is already linked to a Roku account the only way to link it to a different account is to factory reset it.

Grendal 

During setup and activation of any Roku device or RokuTV, just use the same email address as any prior Roku device and that will "link" all Roku devices to that account.

You can see the email address that your Roku account is registered under through Settings/System/About on your Roku devices.

https://support.roku.com/article/360000321787