How do I add extra pages?

You can't add pages per say. The theme isn't in control of the authentication cinematic. The Keycloak server defines the login/registration steps all you can do is customize the pages defined by Keycloak.

That being said, if what you are trying to do is implementing, for example, a multi step registration process you can do this! You'll do it at the page level by dynamically swapping react components.

If you have implemented a custom Keycloak extension (in Java) that does define some non standard extra user facing pages you can implement implement them in your Keycloakify theme. See:

Styling a Custom Page Not Included in Base Keycloak | Keycloakify