There is no tool that will automagically convert PHP code to a PHP Extension, but tools like Zephir provide a half-way DSL that can easily be converted from straight PHP and then compiled against Zephir to build a PHP extension... and yes, it does work with namespaces
The Zephir docs are pretty good, and give a decent explanation of how to write code for the Zephir DSL, and there are blog posts like marmelab's that show how to convert a PHP (namespaced) class to an extension using Zephir.
Edit
Since posting this answer, I've also discovered PHP-to-Zephir which claims to be able to convert PHP code to the Zephir DSL, although I haven't tested it in any way