Appreciate your views on this.
Good idea is first of all to follow yii blog tutorial (if you haven’t yet).
As the next step, the system you describe can be a good studying project. If this system is in a production mode, and won’t be changed a lot, you may leave it as it is, creating the same functionalities with yii at localhost, for example, that would give you a chance to study framework and get accustomed to it. After finishing this project, I think, you will be able to decide if to replace production with a new setup.
Actually, you don’t need or have to use any framework all the time, everything depends on specific needs of a specific project. And if it’s very small and doesn’t require a lot of coding or a lot of specific functionalities frameworks usually provide, it’s normal to go with plain php + js, using a couple third-party classes / libraries if needed.
As for your original question: If your application is really small, porting it to Yii might be a fun exercise. But as Yugene already said, you’d better finish the blog tutorial first.
As a general rule of thumb: The closer an existing application adheres to the MVC pattern, the easier it is to port to another framework.