Speaking purely from the perspective of a C++ programmer, I would think that a Java developer looking to learn C++ would face a fairly steep learning curve. Yes, at some level developing code is similar in all languages. I was once called upon to write code in Excel's macro language (pre-VBA). I managed but it was like trying to write War and Peace with a fifth-grade vocabulary.
There are other issues with C++ (pointers, memory management, operator overloading) which are just not present in Java as far as I know and they're not small issues either. Especially if you're looking at someone to help with legacy C++ code these could present big obstacles. And I think I would start any new hire with at least some maintenance work initially.
