using default depth buffer in off-screen framebuffer
Is it possible to bind default framebuffer's depth buffer to another framebuffer?


First, you "attach" images to framebuffers. You "bind" objects to the context; you "attach" objects to other objects.

Second, no, you cannot attach images of the default framebuffer to anything.


  If not, how can I copy default depth to another depth buff, or what is best way to do this?


You render to your own depth buffer entirely. Just copy/render the image data out to the default framebuffer when its time to display it.