Hi,
There is no such tool to my knowledge. We recently developed a python script doing the reverse operation: converting a URDF file to a Webots PROTO file, I could share it with you if you are interested.
Please note however that most of the models provided within Webots are not licensed under a free open source license. Therefore, you are not allowed to redistribute modified or converted versions of these models, unless you get the official approval of the the copyright owner to do so (copyright owners of the models distributed in Webots include Cyberbotics and various companies and individuals). Please contact me if you have questions about a specific model.
Also, it may be interesting for you: the latest version of Webots, e.g., Webots 7.4.1, include a native generic ROS interface running in all versions of Webots, e.g., Windows, Mac OS X and Linux. This means that any robot in Webots can be connected to the ROS network out of the box, without installing or recompiling anything. See the documentation on Cyberbotics web site (google for ROS Webots).
Best regards,
-Olivier Michel
Cyberbotics Ltd., CEO