It is quite hard (or almost impossible for large robots) to hand code ROS robot definitions in URDF XML files, so that is why SolidWorks has model exporter http://wiki.ros.org/sw_urdf_exporter. SolidWorks has open source alternatives like FreeCAD and Blender. My question is - does SolidWorks CAD-to-URDF exporters works from FreeCAD or maybe other open source CAD programs?
As it is the answer to your question is no. It is developed, by the community, as a plugin specifically for SolidWorks.
If you are a student the fastest workaround would be to get a free student license from SolidWorks. You can then try to export your project to SolidWorks and convert it or directly do it in Solidworks.
Otherwise, for Blender there is the possibility to use Phobos, another dedicated plugin. I am not aware of a method to generate URDF automatically from FreeCAD. And I have no clue for BRL-CAD and OpenSCAD