Thanks David for your help, I was not able to get the Dialogbox in PostNewDocument, so what I did was set a requestScope flag in the PostNewDocument and I called the Dialogbox in the afterPageLoad event and now Dialogbox is showing up with the message, and I executed remaining part of my SSJS code on the Dialogbox Button Bars :) It was little work but not great work.
Please share if you have better way of handling this situation. I will try to get it done using CSJS, but for now this workaround is working fine with me.