Convert them with the editor. You can do that by adding the XML and .X mesh files to the meshes folder of a mod(vehicle) or map(create a meshes folder for the mod(vehicle) or map if required), then upload the mod(vehicle) or map to the workshop. After uploading, subscribe to the mod(vehicle) or map. It will then be downloaded to C:\Program Files (x86)\Steam\steamapps\workshop\content\675010(or wherever Steam/steamapps is installed). Find the folder with the mod(vehicle) or map(the name/number of the folder is in the workshop page URL for the mod(vehicle) or map as id=name/number of the folder). In the main folder the converted mesh file(s) will be in the _m sub-folder.