It depends on what kind of hair extensions you are wearing. For clip on hair extensions, it is hard to do french braid. For tape on and keratin hair extensions, I think you can treat the hair as usual. Just be careful to make sure the bond or tape is invisible.
I am assuming that you know how to make a normal braid. In French braid you mainly start with 3 small sections of hair from the top of your head. Whenever you cross 1 section over the center section you include small amount of hair from the sides. This is how the French braid is made. For more details visit following site.