First, what are extension tubes (one might ask). Extension tubes are inserted between the lens and the sensor. Second, why would you want them? To get closer to your subject for detailed macro work. There is a little bit of skill that goes into using extension tubes properly.
Fuji is coming out with an 11mm & a 16mm tubes. I doubt they will be sold as a set, like most of the second hand tubes that are out there. They have electronic contacts, so autofocus and aperture sill still work.
Check out the announcement: (content copywrite by Fuji)