I don't know if there's anything out there already, but I'd've thought it'd be possible with lots of #define insanity - basically #define around all the differences between the languages and end up writing in effectively a higher level language that abstracts down into the right language for a particular platform.
(So you'd still have to rewrite most of your existing code to fit your new syntax, but once that was done, you'd have code that could be compiled in multiple languages)
The code would end up ugly as hell, and, as I said, it'd probably drive you loopy, but it should be possible in theory.
(I can't find the link atm, but there's a great site somewhere that talks about multilingual code - basically having a source file that's valid under multiple languages by tricking the compiler into thinking the other language's code are just comments / defined out, etc)