[img]http://www.thermaldynamics.com/pictures/guns/m4mre-1.jpg[/img]
The extension is actually a factory part. You can't added it your existing RAS but you can purchase a RAS MRE. This is the exact handguard that is in the picture above. and it will serve your purpose.
One thing, you will have to remove your muzzle device and front sight tower, this means if you have a post ban gun, the installation will have to done by a competent gun smith.