Why would you want to do this? In fact, Greenfoot is BlueJ, or more formally, it is an extension for BlueJ, meaning that BlueJ is running the base framework of the app.

Sry, but i mean to say that most of people have blue j rather than greenfoot(just a prediction). Now if i will share the gfar file then they might not be able to open in bluej. So, do i have to change its extension or i have to do something else; if possible.