Unfortunately, that might not be possible for Americans--the soft-top version of the Golf isn't headed to the U.S. at all, so the R version is likely for European-only consumption, too.
If you're not in the Cabrio fan club, you may be asking yourself, "What? Why?!" and that would be natural--except that Volkswagen recently took the wraps off the GTI Cabrio concept at this year's Wörthersee Tour. With the GTI Cabrio headed to production, it makes sense to drop in the hotter Golf R equipment and build it.
So it goes.