Go into the ... if I can remember ... Menu.rfa and find the ".tga" or ".dds" file for a sniper scope. The scopes are merely an image in the middle of the screen. Use GIMP 2 with the ".dds" plugin, or use photoshop to edit the scope image.
THen, you'd have to go into the "Objects.con" of your sniper weapon and change the scope icon.