Java (the language) -> Parrot shouldn't be any more difficult than any other language (certainlly easier than Perl5, and probably Perl6, too). Java Bytecode -> Parrot is iffy. I noticed that Microsoft ships a Java Bytecode -> CLR Bytecode converter with the .NET SDK, but I don't know how well it works.

Any static language built on Parrot will have the disadvantage that Parrot is meant for dynamic langauges. That's a lot of extra overhead that can be ditched if you're focusing on languages that tend to be static.