You can try to mix standard Android GUI/layout stuff with OpenGL output. Personally, I wouldn't do it, but that might be just a personal preference. If you do it that way, it's independent from jPCT-AE and this forum isn't the right place to ask for support.
The other option is stay within the OpenGL world, but that requires you to re-invent some stuff. For a simple button, you can create a texture that contains the button as an image and use one of the blit()-methods in FrameBuffer to draw it on screen.