You are right. roScreen is a separate screen that in general cannot be used underneath other *Screen* objects and there are no "parts" that can be assembled and used on an roScreen. You would have to create usable parts separately. roScreen is designed mostly for custom UI where you want fine control of the UI and/or better performance of drawing (using direct drawing, double buffering, etc.) with the drawback of the developer needing to write a lot of supporting code.
I have used many of the selector screens in a stack for basic selection, then instantiated an roScreen on top for the "main" application/screen functionality, then deleted to roScreen when done to reveal the *Screen* stack again. Works well, but a little tricky. The
use of roScreen relative to other objects is decribed in a forum post.