Do you mean to transform letters drawn by hand with a brush or a pen? This can not be done with LibreCAD. What is the purpose?
You have to use a different program. You can try Inkscape, which is Open Source and free.
First you have to make a high quality scan, preferably in black and white only, no greyscales. You can try if a photo is good enough. Best save it as png. Then import or open in Inkscape. Go to Path - Trace Bitmap. Then go to File - Save as and select dxf.
There are also other vectorising tools you may google for, I recommend try Inkscape first. It also depends on what you need it for.