how to convert Geo tiff format image in to SAR PRODUCT in snap

Why do you want to convert it to a SAR product? And to which? An ERS product, a ASAR product or TerraSAR or Sentinel-1 product?
But in general this is not possible. Such specific formats are only read. Export is supported for generic formats only.
But if you tell your use case, we might find another way to help you.

sir it is a sample data of terra SAR x single pol . i am interested to classify the image based on back scatter energy

But then you can simply do it in SNAP. Or are you missing something?
Maybe someone else can help?

yes, if you can import the image in SNAP you can simply use it in the classification module as an input.