I agree with Kiran that you can't do a CALL SCREEN from a WDA application. This pretty much defeats the whole purpose of WDA development.
Ideally, you should re-design your application to use WDA concepts and split the business logic from the UI logic (MVC design pattern).
Now, having said that, it could take you a very long time to re-design all of the good work you did in traditional ABAP. So, it is possible to call an ITS version of the ABAP transaction. To do so, you need to create an Object-Based Navigatin (OBN) reference to the ITS transaction. You can call OBN from within WDA methods. The OBN reference then triggers the corresponding object which could be a transaction, another WDA, etc. Keep in mind though that OBN will be asynchronous, meaning that the OBN target will be opened in a new window.