Hi,
Feel free to fork the
github repository and then send a pull request with your converter.
Similarly to e.g. VTK to OBJ converter that is present in PCL, you may want to create an app "vtk2off.cpp" in "tools" module. The implementation itself might go to "io" module ("io/include/pcl/io/off_io.h" and "io/src/off_io.cpp" to be more specific).
Regards,
Sergey