Hi Julien,
With version 0.8.0 (the current master branch), you can run the openmc-xsdir-to-xml script to convert an xsdir file to the OpenMC format cross_sections.xml. Since version 0.8.0 uses ACE cross sections, you don’t have to convert the data itself. Give that a try and let us know if you run into any problems.
Best,
Paul