I suppose you want to use ttf fonts in LibreCAD. This is not supported. There is a command line tool to convert ttf to lff, the LibreCAD font format. You get an outline only. You can explode the resulting text and try to fill it with a solid hatch, works sometimes.
ttf2lff:
https://forum.librecad.org/Using-ttf2lff-for-windows-tp5708647.htmlI didn't get it to work, maybe you will.