The LibreCAD source tree contains a librecad/support/patterns directory which you can add the dxf file to:
https://github.com/LibreCAD/LibreCAD/tree/master/librecad/support/patternsFork the repository on github, commit your new file, then submit a pull request against the librecad/librecad repository. For more detailed instructions on contributing:
https://github.com/LibreCAD/LibreCAD/wiki/Git-and-GitHub