Yes, that works in Perl 6 too.
So, to recap:
> Can one even use CPAN with Perl6?
Yes.
A Perl 5 coder might write:
use Data::Dumper;
print Data::Dumper->Dump([1]);
A Perl 6 coder can now use the same Perl 5 module, eg:
use Data::Dumper:from;
print Data::Dumper.Dump([1]);
The Perl 6 "adverbial phrase" `:from` at the end of the `use` statement line tells Perl 6 what module loader to use.
(Another important `from` option in Perl 6 that's currently being brought up to production grade is `:from`.)
The `.` rather than a `->` in the print statement line is another giveaway that this is Perl 6 code. And the `[1]`; is that a Perl 5 or a Perl 6 array literal? It doesn't matter. This line is a taste of how slick Perl 6 interop magic is. You barely notice it, but the language is automatically marshaling data and objects back and forth between languages as necessary.
Of course, the above is a trivial example. But that's not because complex examples don't work; within a few weeks of starting Inline::Perl5 its author Stefan Seifert was demoing a Catalyst app written in Perl 6. That needed fancy stuff beyond mere passing objects back and forth; in this case Perl 6 code had to subclass a class from another language, in this case Perl 5, and have Perl 5 accept objects made with the subclass as if they were Perl 5 objects made from a Perl 5 subclass.
Inline::Perl5 hasn't yet been smoke tested with all 130,000 CPAN modules, and it will no doubt fail with a few, and a similar story applies for the even more immature support for calling Java libs, but this tech ought to be a game changer, at least within the Perl community.
> Is it even possible to convert existing Perl5 CPAN packages?
There's no need. (See previous point.)
But if you really want to, then yes it is possible, and seems to be enjoyable. Reports I've read of P5-to-P6 conversion stories tend to express satisfaction that basic syntax is generally the same, the changes that are necessary make sense, and the final code is a lot shorter, often half the length, and is very readable.
It's not all roses with Perl 6. There's plenty to bitch about. (#1 is still speed.) But use of CPAN modules, and writing Perl 6 code alongside Perl 5 code, are quickly shifting over to the strengths column for Perl 6, not the weakness one.