How to convert mono to stereo FM transmitter?

You probably mean how to convert mono to stereo FM transmitter.
First of all you need stereo coder. It converts L and R audio channels to MPX signal. Transmitter is modulated with this MPX signal. Usually it is neccessary to increase bandwidth of modulator up tp 100kHz. Allso, the RF filters in transmitter must have at least 300kHz bandwidth.

do u have a sample circuit for it?

For simple circuits you should search Google for pirate radio or fm. I saw, they have schematics. You could search for veronica, or somthing like this, transmitter schematics. Allso schematics for stereo coders exists.

do u have an available stereo FM transmitter circuit that works only on milliwatts or 1W?

Sorry I dont have schematics. Google.