I think you might be going about this the wrong way. First, all OpenGL ES rendering is self-contained within your CAEAGLLayer, so you can't just add a subview and expect it to be part of your scene. The OpenGL ES content will render within a flat layer, and anything added on top of it will just be stacked on that layer.
You could grab an image of the map view, convert that to a texture, and then upload that texture into your OpenGL scene, but that would probably have terrible performance and I don't think you'd be able to maintain user interaction with the map.
If you are just attempting to provide a perspective effect to your map, you could use a Core Animation CATransform3D to rotate the map view in 3-D and apply perspective to it. See my answer here for an example of how that could be achieved. Using Core Animation for this would require far less code than OpenGL ES.