This is the post which help me to get it done. and here i am resharing the tip.
I think extending the facebox is much better than editing the facebox core file. So lets extends the facebox and add one more property to its settings named "modal" which is a boolean.
So now when the modal settings is true u need to avoid triggering the close event while clicking on the overlay.
So to do this u need to remove the default click event on the overlay by unbinding it.