While this is not likely to be true at launch, within a year or so Parrot should be happy to load Perl5 code therefore making the majority (if not the entirety, depending on how XS dependant modules are handled) of CPAN available to Perl6.
Implementing that is definitely not trivial. I think it is more likely that they will end up providing *some* support for XS modules, with the recommendation that we use the new system. Some will not be able to rely upon this emulation layer and it will not move to perl6 until they are really motivated. Continuations and closures alone, won't cut it. If it were so, all those proprietary, never seen by CPAN but running in production environments XS modules, were written in Lisp/Scheme by now