There are several posts on the subject.
The easiest way to have both wheels and do the conversion is to have Warp 9 or Wheeling Cycle Supply or someone else build you a set of 17" wheels with the front being based off the R hub and brake rotor. Then all you have to do is swap wheels and adjust the chain.
Another option is to find a set of stock X wheels, or an aftermarket set based around the X hub and front rotor, and swap to the X front brake assembly. A little more involved as you'll have to change front brakes every time too, unless you have a 21" wheel laced up around the X hub/front brake, and keep the bigger X rotor and caliper on all the time.
Personally, I'd find an X owner who wants to swap wheels and front brakes, do the swap, and have Warp 9 build me a set of dirt wheels based around the X setup. The stock X hub is very cool, and the bigger X front brake is so much better than the R brake.