Hello Arnab,
I was searching for the same question when i came across this thread. Doing some analysis i got the answer. I am not sure its helpful to you as the thread is from year 2012.
For your scenario when you need to pass the data in the pop up from an FPM, you must be using
cl_fpm=>get_instance( )->open_dialog_box to open the pop up.
If you check the import parameters this contains io_event_data as one of the importing parameters. This will be used to pass the data to the pop up.
Create the instance of this object and use set_data method to pass the data.
Now the question is where you can catch this data. If its a free style UIBB then you must have implemented an interface if_wd_ui_building_block.
In the before_output method you will get this passed data in io_event object.
There you can access the passed data, using mo_event_data->parameters.
I wish the solution will be helpful to someone who come searching for this.